I'm afraid you'll have to back up all your data and reformat the whole disk Apple utilites won't let you repartition the disk keeping any other partition untouched.

As for thrid-party utilities, my only experience is with FormatterOne Pro; I think it can do what you want, but only if the disk had been previously formatted with it... most likely other utilities behave the same, but I'm not sure.

Anyway, it's always a good thing to have a back-up copy...