I just bought some bones

Is it possible to make a mailman conversion spell out of them. Asking for a friend.

I believe Mailmen can be converted into bones, but not the other way around.
Bones are, however, good for attracting dogs, which are the natural enemy of mailmen. They are naturally equipped to hunt them down and can be well bribed with bones.

sell them for 15gold a pop to adventurers and orphaned babies who happen upon your doorstep.

15 gold is nothing for a seasoned adventurer but quite steep for a penniless baby. Consider offering orphans a small discount to appear more sensitive and human to media outlets.