if you can import said models into 3d programs like, say, 3dsMax, and if said models/meshes have the same bones as that of the standard UT2004 models, why not.
Although if the bones don't match though(assuming you can already import it into 3dsmax) you can still import it in UT2004 on UnrealEd, although you would have to make all of the anims for the model(e.g. walkf, runf, crouch, Biggun_Burst, etc)
Originally Posted by Sly.
But the other question is: Is it allowed to convert it?
You would have to check the game's EULA.
all this importing/exporting is good and all, but you should check if it's legal first(if that's the proper term)