Might work, might not. 5 days difference may be too much. Make sure the new babies are active and eating. Some hens don't mind extras, some do. Distract the hen with a favorite food, then put in chicks when she is busy eating. Or wait until evening and slip them under her. If you do it in the day while she is eating you can at least be around to watch and see what happens. if you do it at night be there first thing in the morning to see what going on. Remove chicks if any are getting pecked or if they won't go near the hen. Puttting in more than one is a good idea, if there are problems one chick probably won't get all the punishment. They should want to seek out a hen if they were hatched under one, so they shouldn't get cold if she'll have them. Its a bit different with chicks hatched in incubators.
