No. Room-only is just that, room only. To obtain the dining plan you need to book a vacation package with at least a one day basic ticket. Cannot change from a RO to a package upon check in. The systems are different too and monies cannot transfer from one system to another (and don't let anyone tell you otherwise).
If you want the dining plan you must rebook a package with a ticket, and then do a $200 deposit. Then cancel the RO reservation. Package reservations must be paid in full at least 45 days prior to check in, while a RO may be paid off upon check in. (You have a one night deposit with a RO).