It's also possible to "rob the pack", remove only the exposed negatives and leave the unexposed ones -- this should be relatively simple, since the torn-off paper will distinguish one from the other, and the exposed ones should be right on top when the pack shell is opened. The tricky part might be opening the shell and reclosing it in the dark. If you have a camera that the pack film back fits, it's probably easier to go ahead and expose the remaining film.
Plus, of course, that pack film is thinner and slightly larger (length and width) than the same nominal size cut film, which can make it interesting to process...