I'm doing just that thing for my trip in two weeks. It was a nightmare trying to get it done through Disney though. It took 4 phone calls, with several directly to DME before it got done. I arrive on a Mon morning, dh and dd arrive Tues night...shouldn't have been tough to do. I called Disney to make our room reservation and then added DME...was told that we couldn't do it that way, that everyon had to come in on the same day. No, I told her, we could come in on different days. So, hung up and tried again...same thing, but at least this person tried to do it, had to call DME herself and 'said' they talked her through it. Now, I knew it could be done because I had called DME just to double check after the first call and the DME rep just howled when she heard the DIsney reps were telling people it couldn't be done. So...the second rep said it was all done. But, when I had to call back to change something, (about a week later) the rep asked if I wanted to add DME!!!! Said I already had it...no, he said, you don't have it listed. So, went through the exact same thing again!! He had to call DME directly, who gave him instructions as to how to put it into the system. Okay....good to go? Nope, didn't work that time either. Finally, I got a rep who had a handle on it, she called DME yet again, it did get done corrrectly this time.
I did hold my breath until my tags came in, but when they did,,,,all info was correct!!
It seems that the system doesn't easily take different arrival dates. Call and make the reservation, but if I were you, I would double check just to be sure it gets done correctly!! Then, about a month before you travel, call DME directly to see what they have in their system.