Duncan cut the hand off Xavier so we know from that a limb can't be regrown
Whether you could hold it in place and have it magically reattach is another matter
If you look at the limits of a healing power, it is different between Highlander, Deadpool, Wolverine, and Clare in Heroes
Clare can regrow a severed part
Wolverine had his legs ripped off and reattached
Deadpool was ripped in half by Juggernaut and slowly regrew the bottom half
the higher limits of the healing power are not regularly explored in any of the shows or stories, so the writers would have to decide how it works when they get the chance to go there
With Xavier, we see him come back with the human mercs who gun down the immortal and drive off so he can take their head
He can no longer sword fight so invents this "cheat" method
It's a good storyline and relies on the concept being a severed hand can't be regrown
