Up until now, creating Procedural bone data required SFM, which is a pain to use.
The developer of SourceOps, bonjorno7, added functions to his blender tool to do the same.
Get the addon on this Github page[github.com]
(Made for 2.92.0 but It also works on 2.8)

The following guide assumes you know what procedural bones are.
It also assumes you already have properly rigged helper bones ready to be used.
If you do not know what procedural bones are to begin with, check the VDC

WARNING:
If you are working on a model with proportions, you should do this after you are 100% sure that you are not going to edit the proportions any more!
Editing proportions requires you to redo the VRD from scratch.

To install the addon, open user preferences by using Ctrl & Alt & U or "Edit > Preferences".
Click the "Install" Button, Find and doubleclick the SourceOps.zip.
Enable the addon by clicking the Tickbox.

You need a VRD file called "Procedurals.vrd" in which the procedural bone data will be written in.
Just make a txt and change the file ending, or make a new file in Notepad++ and save it as Procedurals.vrd
If you don't have any base, use this:
This uses three triggers, as i'm working on a hand. Other bones like elbows only need two.
You could use as many as you want though.

Your QC file will need the following line to read the VRD data:

Write bone names for the "Helper" field.
Those being: the procedural bone, its root, its parent (same bone) and the control bone.
Note: The ValveBiped. part of the full bone name is removed.

Select your helper bone in pose mode and bring up the "Context Menu".
If LMB is assigned to "select", context menu assigned to RMB.
If RMB is assigned to "select", context menu assigned to W.

Click "Translation for Source".
The required numbers are copied to your clipboard, ready to be pasted into the VRD.

You can see the default letter assigned to this action is "A".
Next time you could use RMB followed by A, or W followed by A. Gotta go fast, amirite?

Paste the numbers into the "Basepos" field.
You may notice that the two last numbers contain "e-": -3.4e-05 1.4e-05.
Those are e-notations[en.wikipedia.org], which source engine understands. Ignore them.

The first trigger is the "Rest" trigger.
Make sure your pose in blender is the rest pose.
Select the Control bone of your helper, which is the left hand for me.
Bring up the context menu again and pick "Rotation for Source"
Or the shortcuts RMB followed by O, or W followed by O


Paste the new copied numbers into the first trigger, after the 90:

Select the procedural bone and copy its rotational values right after the control rotation values.

Next are the translational values of the procedural. Copy yours if you use them.
I have not moved it, so i just use 0 0 0.

This step is optional. You do not need it, but if you are unsure about making the poses, you should consider following this step.

You'll need to pose the helper bones in at least three positions: Base, positive, and negative.
You could turn on "Auto Keying" in order to temporarily keep the poses of your helpers.
This allows you to jump back into blender and adjust the poses, if you feel like you need to do so.
Not having these keyframes means you'll need to redo the poses from scratch.

First, enter the animation screen.


Press this little button. It looks like the "Rec." LED on camcorders.


Enter pose mode, Press A to select all bones.
Then press i to insert keyframes for all bones.
Pick "Whole Character"


The "Dope sheet" will now show a whole bunch of dots. Those are your keyframes.

If you did the Keyframe setup, you will now need to press the Right Arrow key.
the Dope Sheet should now show a "1" at the very top.


Now that we have a base, we need a trigger for an extreme rotation.
Pose the hand and wrist bone as far into one direction as it would be humanly possible.
I rotated my hand by -100°, wrist by -80° and the forearm helper by -50° on X.
The Forearm needs its own setup, for now i'm posing it to make sure the entire arm looks good.


Now do the same three steps as you just did on the base trigger.

Select the Control bone of your helper, which is the left hand for me.
Bring up the context menu again and pick "Rotation for Source"
Or the shortcuts RMB followed by O, or W followed by O

Paste the new copied numbers into the second trigger, after the 90:

Select the procedural bone and copy its rotational values right after the control rotation values.

I have not moved by procedural bone, so my translational values will remain 0 0 0.

Checking up on the Keyframes, you should now have a second row of dots.

If not, you didn't activate the auto key. So you could just select all bones and inserting keyframes for all bones again.

Press the Right Arrow key once more to advance to frame 2.
So we won't overwrite the pose of the negative pose.


We now have triggers for base and far negative rotation. We still need far positive rotation.
Undo all poses and pose all bones by the same amount in the other direction.
I will rotate my hand by 100°, wrist by 80° and the forearm helper by 50° on X.


Write down the rotational data in the third trigger. with 0 0 0 transition, because nothing moved.

You now should have another row of dots in the Dope Sheet.


By the way, you can use up to 32 total triggers per helper bone, should you need more poses.
For example, maybe if your wrist is curled inwards

Repeat these steps for all procedural bones and compile your model.
You may notice the ulna block being the wrist block, but with changed basepos and rotational values of the procedural bone in all triggers.


If you feel like your procedurals are not precise enough, you can add more triggers.
Instead of three triggers you could have five triggers (base, 50% -, 100% -, 50% +, 100% +).
Additional triggers in between are usually not required, but maybe you might need it.

Congratulations, you now have kept the helper bone poses. You may jump between frames using the left and right arrow keys, allowing you to alter the poses and gather new data without having to redo the entire pose.

If you want, you can keep this "animation" forever in your blender save. You don't need to though.
For this, swap the Dope sheet to "Action Editor"


You may need to scroll to the right, by hovering over the controls of the dope sheet and scrolling your wheel, until you get to the text box. Name this thing VRD.


You can now just scroll back to frame 0 to go back to your ref pose.
You can also disable "Auto Keying", and press the little X next to the VRD action name to drop the animation from selection.


Now you can come back to this at any point.
Granted, if you have a proportion trick going on and you move the entire proportions, you will need to redo all of this from scratch.