There is a large branch of Sansoucie's in missouri. Check out the sansoucie board on here. You can fins a lot of info. I have never heard of Melzina, but check with some of the Sansoucie researchers there.
I married into the Sansoucies, but have much info as I am the keeper of the family records and bibles. I will check on my end and look for a Melzina. It wasn't uncommon for the French to marry into the local indian tribes and your family lore could well be true. On my father's side there was a tale about a "black Dutch" wife and it turned out she was full blooded blackfoot. Keep on searching!!