In all reality, all you'd need to port over is the head of the companion of your choice since you can easily assign one of the many different body types FO3 or FNV modders have made (unless you want a custom body in which you'd need a custom skeleton as well). You'd of course would have to mold the the head to the proportions of the body.
Then there's the issue of face rigging which would allow the companion to make facial movements: speaking, emotions, etc.
Adding voices is probably the easiest part in all of this though.
"A drop in the Basket, Is Worth a Shepherd's Shilling" ~ Danka Chevchnov