It would help if your clarified your question. Do you mean modelling or coding? If you mean codingthen its the exact same method you add any other unit.
or copy a infantry unit rename it and give it a new skin so that it is a new unit that still moves like it shuld
I don't think anyone has been able to do this yet. My theory is that once infantry models are put in Gmax they are corrupted somehow and will not use their animations. I hope I'm wrong.
i'll point in the direction of the renx wwskin space warp tool, there's a tutorial that came with the renx gamepack to show how to use this complicated beast of a tool