I only have Elves and Humans to kill in my map and would like Goblins and/or Kobolds to show up as well. My embark site doesn't have those features, but I'm on a really awesome map otherwise and would just like to go and hack some files to get more sieging.
IIRC, if civilizations are dead, you can't interact with them.
This being said, if you use DFHack, Putnam made a script that "forces" a certain event to happen. The script can be downloaded from DFFD. Just place the compressed file in DFHack's
To create a goblin siege, write in DFHack's terminal:
force siege EVIL
(EVIL being the goblin's ID ingame). The script is finicky and the siege may not appear immediately, but if I remember correctly, it should create the event. The script can also force other events, but as I said earlier, it is finicky.