Yes poop coming out your belly button is possible but very unlikely and it's rare that someone has poop come out there belly button.
Your belly button is sealed up so there should be nothing coming out of your belly button unless your belly button comes open somehow.
I've heard of a few cases where some people experienced poop or fecal matter coming out there belly button but it's rare that it happens.
Poop can sometimes come out of a persons belly button due to the umbilical fistula, an abnormally developed passageway between the intestines and umbilicus which causes the poop to come out the belly button.
You're very unlikely to have it happen though although it happens and is entirely possible.