I know not all schools that teach Hapkido will do this but I once knew of a school that taught both Tae Kwon Do and Hapkido. In order to train in Hapkido at that school first you had to get a black belt in Tae Kwon Do there. I do know that Hapkido does use some of the techniques from Tae Kwon Do but to even start as a white belt in Hapkido at that school you first had to get a black belt in their Tae Kwon Do. I wonder if other schools have that same requirement.