Using KSE is basically equivalent to using them as a disguise. Since they are full body models, a disguise is probably the most expedient route for a mod, albeit one saddled with a few potential issues.
If you really want to make a version that turns up in the character creation menu then you'll need to split out the heads to their own model, make provision for them in heads.2da and portraits.2da, etc. Look at an existing head mod for an idea of how the setup works. The main difference would be that you'd need to set every body model to their matching custom body, not the regular underwear/clothes/armour/robes models.
Note that since character creation ignores the head's appearance.2da row, the head will be slapped on the appropriate class outfit in K1 and commoner clothes in TSL for that bit.