I tried making a new fortress, copied the .bse file for wild using worldbuilder and renamed it, but I couldn't get it to use my new commandset.
So, I went to commandbutton.ini and changed the command to build the builder by adding 'needsupgrade' to the options line and adding the line neededupgrade: upgradegoblinfaction, (if I remember correctly) for thegoblin builder and the same sort of thing for my new builder, and in commandset put a command for each builder in the fortress commandset. That worked, now each faction can only build their own builder, though the other one's button is visible grayed out. I mentioned that just for the sake of anyone else who might read this looking for help.
I would appreciate a tutorial on how to make a new fortress if there is one available.