I believe you can, but I think once all the LORs have arrived to AADSAS that correspond to the list of LORs you've entered in, then they will send it immediately. However, you can also send LORs that you have not indicated, but just remember to put the LORs matching form so that they can match it up to your file.
However, to be safe, you might just want to send the extra LORs directly to the schools.