You can only add part of the information to make this a continuing reservation. The phone CM can easily make a note that this new (1) night reservation continues to your Agoda reservation if you book directly with Disney. Do you have the Disney reservation number for your Agoda (2) nigh reservation to provide to the phone CM.
If you find an available room on Expedia you can also book there but you really want to make sure both reservations are in the same category if possible. Your real issue is that you can't actually fully link both reservations when part of your (2) reservations are owned by a third-party until you get to the resort.
You also need to stop by the front desk and let the CM know you have (2) reservations in what is called a continuing reservation, and that you want to stay in the same room if possible. Once you get to the resort the Front desk CM can update both reservations and link them since the third-party restriction is gone.
It is a win win situation for both you and the resort. They don't have to do the extra servicing of the first room, and you get to stay in the same room.
Dave