I hope you do understand that Texmod simply changes the texture of the 3d model. There's not some magic going on that will completely change the physical appearance or geometry of the character. Applying Miranda's texture to Kelly's 3d model will probably lead to unsatisfactory results. To my knowledge, you can not inject 3d models into the game the same way that Texmod replaces textures.
You can however change models for certain characters by changing variables in Coalesced.ini, although I am not sure if you can easily change those of NPC's as well. By doing this, you can fool the engine into loading other content packages. (like loading Miranda's 3d model instead of Kelly's) Take a look at these topics at the bioware forums: http://social.bioware.com/forum/1/topic/128/index/1180449/1
Something that might also complicate the process: Kelly's head consists of a seperate hair model and a generic head model. The head model is shared by many other characters in the game and customized per character using morph targets. If you were to replace Kelly's head model for Miranda's, the game might still try to apply the previously mentioned morphs to Miranda's face; which could deform her face into a weird shape or otherwise bug out the game.
You'll probably have to dive in and experiment with some of the settings. Changing the settings might also lead to corrupted save files and game crashes, so be sure to back those up. It will not be as easy as just applying the skin through texmod or some other program. I honestly think it's more trouble than it's worth.