Use this:
https://www.nma-fallout.com/resources/fallout-2-proto-manager.73/
Add the *.frm file to the correct *.lst file and it will show up in the tool. Or just use the BIS Mapper and assign the frm in there.
The critter.lst file in critter.dat is the only file I've found so far that has an obvious reference to the .frm files, but I haven't been able to figure out what the numbers stand for, and adding a new .frm file without those numbers doesn't appear to do anything on first try. A google and forum search have only produced this thread, so I'm not sure where to look for more specific info.
None of the tutorials in this list cover actually adding new .frm's to the critter.lst file:
https://falloutmods.fandom.com/wiki/Category:Fallout_2_tutorials_-_art
This tutorial mentions "updat[ing] the *.lst file", but gives no specifics as to what the extra numbers mean, nor which file or how to update, nor generally how:
https://falloutmods.fandom.com/wiki/How_To_Use_FOT_Critters_In_FO2
I've been through the mapper documentation and can't find anything detailing how to add new .frm's to a critter or an item, although I haven't yet tested librarian mode...and the Mapper Keys documentation stickied on this forum appears to be the only thing I've ever seen referencing how to set up and use this mode, but is very limited in it's descriptions. Is this mode what you're referring to?
Finally, I can't find anything explaining how to add .frm files to a non-critter proto, nor can I find any other .lst files that appear to reference .frm files except intrface.lst, and that one is definitely for the interface.
If you've got the time, would you please point me to any resources or documentation explaining how this process works? (even in Russian or German - google translate isn't perfect but it does help)