My opinion, though not informed by the Paranet Papers as I haven't gotten them.
"Having" a stunt, as in it is from now on on his character sheet, a definite no.
As a one time deal? That's basically like spending a fate point on an appropriate aspect, only you pay for it later through debt rather than getting it by a compel. That seems fine.
I could even see getting a stunt temporarily as per the temporary upgrades rules, only again paid with debt instead of a fate point. I wouldn't allow this as a regular thing, though.
Then again, instead of all that, all that might be needed is a skill replacement spell, that allows you to do something at a greater skill level than you otherwise would.
Lastly, the stunt in question would obviously need to fit the sponsor in some way, and it should make sense that it is somehow transferable.