VendorX released a tool that extracts Unreal 2 models from *.gem files and converts them into the standard Unreal skeletal format, so they mostly work in UT2004/UT99/Unreal 227. I have the Unreal 2 Skaarj models converted, but for whatever reason their origin is messed up. They appear to be standing upright in 227's model viewer, but when I summon them in game, they're facing the ground and basically floating in the air. Once they start to move, their origin is fine. Changing the origin doesn't fix it, and I've never had this problem with any other skeletal models.
Things like the weapons would be difficult or impossible to convert. In Unreal 2, the first person hands, the main part of the weapons, and other parts are all separate. I haven't found a way to combine them without ruining their animations.
The only creatures that wouldn't be easily converted are the Drakk, since a lot of their animation relies on code (I think).