How would you do this?

First you would need to extract that persons head skin .tga. For example one of the pc heads would be PFHA01 found in the "ERF's" - "swpc_tex_tpa" - "P" branch of the Kotor II section of Kotor Tool. Or N_TaliaH (for the queen of Ondern) found in the "ERF's" - "swpc_tex_tpa" -"N" branch.

You would then need to reskin this texture to add your darkside transitions. A common practice is to name the finished file the same as it was pefore only with a "d" for darkside after it, as well as a number for the stage of the transition. For example "N_TaliaHd1.tpc"

Finally you would need to open your Heads.2da and add the name of the skin (minus the .tpc file type) you wish to use to the "headtexe", "headtexve", "headtexvve" and "headtexvvve" of whichever head you wish to add transitions for. The "v"'s stand for "very" so the more v's the more evil the transition should be.

Final note: Some head models do not support reskins without Hex editing them. I doubt they can use darkside transitions at all, but I could be wrong. If this was the purpose of your question, sorry for wasting your time!

Very carefully...by editing heads.2da

How to set it up for TSL (which I assume you're doing this for):

head: original model name(stays the same)
alttexture: (If you're using a custom neutral head, change this to whatever you named your neutral head)
headtexvvve: (Change this to what you named your 2nd ds transition)
headtexvve: (Change this to what you named your 2nd/final ds transition)
headtexve: (Change this to what you named your 1st ds transition)
headtexe: (Change this to what you named your 1st ds transition)
Example:
head alttexture headtexvvve headtexvve headtexve headtexe
N_TraskH FK_PMH01 FK_PMH01D2 FK_PMH01D1 FK_PMH01D1 FK_PMH01D1

Hope this helped you.

EDIT: If you need further instruction, see my tutorial (http://www.moddb.com/games/237/star-wars-knights-of-the-old-republic-ii/tutorials/26253/new-playable-character-tutorial) for reference.