we had a fifth in a cbr room, and wdw (or at least cbr desk and mousekeeping staff) knew it, because we told them, and they brought us a rollaway (for $15/night)
at the time it was dh/i, dd 6yo and ds 4yo (not in a pack 'n play) and we were there for nine days and around the 7th day my 18yo niece joined us, we told them she was there and requested a rollaway... i did not know about occupancy limits at the time, so it did not occur to me to stay elsewhere due to her 2 night visit... nor would it ever occur to me to "sneak" her in...
ergo, moral of my story is, call and ASK!