Rhetorical question: If they haven't learnt how to support yet, do you think they will be a good carry?
Had a guy who said he wasn't that great at supp, that he got filled, so I offered to swap mid. He said it was his preferred role. He just completely stunk.
That's when I realised: "Hey, hold on. What made me think that this guy who can't support can play any other role - no matter what he believes?"
Do not fall for the "I got supp fill" guy. He won't be competent anywhere else either, so do not give him a carry role.