I've never tried, as I don't have a 7 string, but I'd assume that you'd use a banjo string or one of the high ones from a set of twelves.
at least on 08 if you're using 10's, but what set are you currently using?
Yeah, then an 08 would be worth a try, or even a 07 or 06 if you can find them. But you're looking for something high if you want to go that high.
Remember that Steve Vai didn't take that route when he had Ibanez make the first solid body electric 7 string released as a production model, because the high A would snap easily. You can try it though, shouldn't hurt anything.