I would never recommend just doing a patchwork repair of a wood shingled roof. You need a new roof, period. Given that, it IS possible to sneak some in there without ripping them all the way to the ridge. Once you curse about 459 times from the pain in the butt of using one of those special pry bars that you hammer back at your hand to (try to) pull out the offending nails that are in the way, then cut your replacement shingle, shove it up into place, about 3/4 of an inch shy of where it will end up, then put in your two shingle nails (galv. box type) but aggressively angle them up toward the ridge and put them in JUST below the bottom of the next shingle course above it, then take a scrap of wood and wack the shingle up into place and it will hold. You may have to use a larg pin punch to tighten the nails a little more. I have done this many times and it works fine.