Go to your JA folder and find your way into gamedata/base. Open up your assets1.pk3 and go into ext_data/npcs. Find a npc you want to copy the stats of and move that .npc to somewhere you can keep track of (I usually just throw this crap onto my desktop).
Ok, find the .pk3 with the model you want to turn into a npc. Open it and go into the model folder. There you might find more than one folder. Pick the one with the model/skin you want to use in it and write down the name of that folder.
Go back to the .npc you copied. Right click and and go to properties and make sure read-only is unchecked. Now open it. There might be more than one npc types in here. Delete the sections of the npcs you don't want to use and keep the one you do want to use. Now find the line playermodel and change the name to the name you wrote down. Now go to the name of your npc at the very top and change it to a unique name you want your npc to be called then write that name down.
You can edit and play with the stats on your .npc as you see fit. Id suggest exploring many .npc files and finding things you might want to add to your .npc to make it like you want it and not just a copy of something else. Once your done save and close your .npc and rename it to whatever you want.
When your done make a folder in your gamedata/base called ext_data. In ext_data make a npcs folder. Put your .npc in your npcs folder. If you submit your map anywhere make sure you put your ext_data folder in your .pk3 with your map and release the model with it.
In GTK radiant to spawn your npc make a NPC_Spawner. Right click and look for it in the NPC section. Press N with your NPC_Spawner selected and type npc_type in the key section. Under Value type the that unique name you made earlier. This is not the name of your .npc file but the name of the npc you wrote down. If you open your .npc you'll see it at the top.