If you have dining for the rest of your reservation than a night without dining requires a separate reservation. However, you can have the reservations linked and they will most likely keep you in the same room the whole time. (if of course your extra night is at the same resort)
However, the next day you'll technically have to check out and recheck in, even if they keep you in the same room.
Magical Express isn't an issue. The CM you talk to about the extra night should be able to set it up for you.
You can even set up the extra night at a different resort if you choose, but that makes for a semi-complicated transfer the next day, so you may not wish to do that.
In such cases, if you can afford it, sometimes it's fun to book a higher level room for the extra night. If you're already staying in a Deluxe resort this is less complicated.
Others like to stay in a lower level resort the extra night in order to save money, but it's really your choice.
