Go to Steam/steamapps/common/GarrysMod/bin. Here you should find a .exe called "gmad". Send this .exe to your desktop as a shortcut.Then go to Steam/steamapps/common/GarrysMod/garrysmod/addons. There you should find all of your addons. Select the playermodel (addon) you want and copy it to the desktop. From there, drag the addon to gmad and it should convert the addon into folders.

If you need to, rename the folders. For example: instead of models/player/someguy/playermodelnamehere.mdl, you could instead use models/npc/someguy/npcnamehere.mdl. This can prevent conflictions with the playermodel that still sits in your addons folder.

Launch Crowbar. If you don't have Crowbar, just search up "Crowbar" in the community hub. There should be a download link there. Anyway, once Crowbar is launched, make sure all settings are default, then proceed to browse for your addons folder. Once found, go to the playermodel (usually in a models/player/playermodelnamehere.mdl sort of line up in the folders) and click on it. Once done, click on the "Decomplie MDL" button. Boom! Done with decompiling! It should have created a new folder where the mdl sat called "decompiled 0.19".

Go to the newly created "decompiled 0.19" file. There you should find a qc file. Open the qc file up with Notepad++. Scroll down til find a line along the lines of $includemodel "m_anm (or f_anm).mdl". Hopefully you still have this guide up because what you need to do is backspace all that info out and then copy and paste this in it's place: $includemodel "humans/male_shared.mdl"
$includemodel "humans/male_ss.mdl"
$includemodel "humans/male_gestures.mdl"
$includemodel "humans/male_postures.mdl". Bare in mind, those lines can also be female, just to let you know. Once done, save then compile the qc in Half Life 2. You can find your newly compiled model in steamapps/common/half-life 2/models. Take that mdl out and put it in the same place where your adoon's folder originally sat (in the models section of your addon, specifically where the mdl sits). Now your done, well not quite...

If i were you, i'd simply convert an addon that has a npc lua file then rename everything in it to fit your npc. Like the category it's in, what type of npc it is, how much health it has, etc and etc. If you don't understand, just comment in the comments section.

I'm pretty sure this would work the same for npcs to playermodels. Instead of using those previous $incudemodels, you would use these: ($includemodel "m_anm.mdl"
$includemodel "m_gst.mdl"
$includemodel "m_pst.mdl"
$includemodel "m_shd.mdl"
$includemodel "m_ss.mdl"), or these: ($includemodel "f_anm.mdl"
$includemodel "f_gst.mdl"
$includemodel "f_pst.mdl"
$includemodel "f_shd.mdl"
$includemodel "f_ss.mdl").