I guess that would vary from texture to texture, some are generic enough to be usable in Hexen and a bunch of Heretic textures are already in KRPG.
How much work would it be to add all not yet added Heretic textures to the KRPG wadfiles? If all were added, we could just convert a map, leave all textures as original and then replace whatever we want by Hexen textures.
Independent of the texture question, I'd suggest that we all look into the episodes 4-6 in Heretic, and make a list of maps we would want to use for KRPG. Together with the 3rd party maps we already have since SE Alpha, we might have a high enough number to plan the whole storyline through them.
One more idea would be to scour the web for more free-to-build-upon wadfiles out there with maps that could be used as locations.