Hmm I always thought those are skeletal anims...
Yeah the ones for the player characters are skeletal anims, the ones for the monsters in invasion are the original vertex animations. Theres 2 entirely different sets, which means, even with the skeletal rigs there would need to be new animations added. Setting up a rig, especially extending on the UT3 one sounds like the easy part, the hard part is the new anims.