If you want to give an existing UT2k4 skeletal models/meshes new animations the only way to do it is to use their existing skeleton (the source art files are out there somewhere) and animate those.
You can then import just that .psa skeletal animation file into either a .u (via #EXEC directives) or a .ukx file (via UEd).
Actually using the new animation can be tricky. If you are subclassing xPawn, then by adding a simple line like "LinkSkelAnim(NewAnimationName,)" to the PostBeginPlay function the pawn will then have access to the animation. Then you can use just a normal PlayAnim call to play the animation.
However if not creating a new pawn class, then depending on what you are trying to do with the animation, you may have to code a more elaborate work around. For instance, lets say your new animation was a new taunt. You could use a mutator to spawn a ReplicationInfo subclass that allows one client to call a "mutate DoNewTauntMove" etc. that then lets all of the other clients know that the one pawn wants to perform a new taunt move. If the client trying to play the animation is relative them (!=none) they should have that copy of the pawn link the new skelanim to itself and then play that animation: ClientPawnCopy.LinkSkelAnim(NewAnimationSet,); ClientPawnCopy.SetAnimAction('NewAnimationName');
So what exactly are you trying to do with the new animations?