If you're lucky, you probably won't have to rig. That's only if they have the same skeleton.
1. Decompile default Blue Shift barney in a temp folder on the desktop
2. Decompile your custom barney in a different folder
3. Drag all the default animation .SMDs from BS to your custom barney. You may need to sort by file type to do this. If I'm right, it starts with "idle1" and goes past "buttonpush." I'm looking at /valve/ barney so I'm not sure.
4. Open the default Blue Shift Barney .QC and look for // " [number] animation sequence(s). Depending on the model, you may have more or less sequences. Select all of the $sequences. Since they should be the last lines on the .QC, it might be safe to say just copy the line // animation sequence(s) and everything below it.

5. Go to your custom barney, open the QC, and paste over the animation sequences there. Remember, don't just paste it all after, it should replace the custom model animations (which need the BS animations you say)
6. Compile it (the custom model that needs BS anim,) then see if it loads into HLMV or whatever you use
Hope I was clear enough, it's 9 PM here and I'm a bit tired. Anything else?
Also, the same might apply with other NPCs.