I've run into this as an option in another game (name escapes me off hand) where one could take actual physical effort and apply it to one's magic.
So, in DFRPG terms, that would mean allowing a character to use her physical stress track as well as magical for spell-casting.
What level of refresh would you require for that? My immediate thought was -2 or -3 - I don't see going much higher because in any conflict, adding physical stress could backfire big time - but I don't see it being cheaper because it does increase their spell-casting, even if only in theory.