I don't see why you couldn't do that. If you could make use of the extra meal credits, it makes sense. Especially if you're booking a room at a value resort.
We actually do the opposite. If we're going for 6 days, we book two reservations. One 5 night reservation, with the dining plan and then a single night room-only reservation. This gives us 5 days of the dining plan and 6 days to use the credits. DS5 doesn't eat a whole lot, so the 5 days of counter service credits usually last us 6 days.