We did just that back in May..it was a pain to get it right, but it did work out fine. I checked into POP on a Monday morning. Dh and dd were to arrive late Tuesday evening. I booked through Disney, and gave them the info they asked for. THey had the hardest time getting it right in their system. They told me the first time I tried to do it that I couldn't do it that way...everyone had to arrive at the same time. Ah, no, don't think so. So, I called DME directly, they laughed and told me that yes, CRO could do it, but if they had an issue they should put me on hold and call DME directly, who would then 'walk them through it'!!
It took me 4 tries to get it in the system correctly. But, it did work out. THe luggage tags all came with my arrival date on them. But, since dh and dh weren't checking luggage it wasn't an issue.
I would just keep at them. If you want to be sure it's done correctly, call DME directly about 5 weeks before your trip and see what they have listed for you in their system...that's what I finally did.