There is no reason why he cannot be. Muramasa seems ownerless and Kenpachi does not know or even care about the name of his sword. What makes him perfect for Kenpachi is the fact that his special ability is to appeal to the instincts of the other. Which in a fight could bring someone down to the rage in Kenpachi and turn any fight into a berserking brawl of psychos. The biggest thing that has me thinking this, is that Muramasa seems as though he is actually looking to be accepted and not so much just out to destroy the Sereti. I mean techically he doesnt control the other zanpakutos otherwise they would have to kill Muramasa to break the hold. Not only that but when he became weakened after fighting Ichigo he should have most likely lost all control of the spirits and the series was over i think the zanpakutos are actually all being little children looking to be accepted by their fathers in this Arc and Kenpachi is the ultimate father who never even cares that his son exsists. if any of you think i am crazy let me know i dont care if you do but i just want opinions.