if you have milkshape and fragmotion this can be done easily,here are the steps useing the two programs-
-Part1~Fragmotion-
1.import the tommy.x into fragmotion
2.export tommy as a .ms3d file
3.import zombie of choice.x file from mp4
4. export zombie of choice as a .ms3d file
-Part2~Milkshape3d-
1.open zombie of choice.ms3d file
2.select every vertex and erase the model
3.export it as a halflife .smd animation file
4.close that model out and open the Tommy.ms3d file
5.import the zombie of choice.smd file;it will ask if you want the animation apended at the end, select no,this way it overwrites the preveous animations.
6.save the new TommyZombie.ms3d file just incase something goes wrong.Always take precautions.
7.Use milkshapes JT .x exporter to create the new Tommyzombie.x
There ya have it.Milkshape and fragmotion compliment each other very well and usually if milkshape cant do it fragmotion can help it along. Hope that helped