As of right now, you can't. You can only modify a current reservation over the phone.
One thing to note about making these plans now. When you book a room-only for a time period that the rates have not yet been released for, you will be quoted this year's maximum rate for the room type. This means Holiday pricing/Weekend Rates. The deposit required will be one night stay on these rates. The total cost would be changed to reflect correct pricing once the 2011 rates are released to the public (sometime in Aug).
What the OPs process will be is:
1) Book Room-only for dates right now. Pay 1 night maximum price for room deposit.
2) Wait until August or so until pricing is announced
3) Call Disney and modify reservation to package. Note that there is (at least as of this year, it could change) a 200$ deposit required to book a package. If this is higher than your room-only deposit, you will be required to pay the difference at time of booking. If lower, they can apply your room-only toward it.
4) If any promotions come up for your travel dates, call Disney again and check if a) there's availability and b) what the cost will then be. They will apply discount codes to existing reservations if there's room and if you're booking would qualify. Also note that only one discount per reservation will apply.