Kevin, it should be no problem at all if their system works as it did for me last year.
I added the 10 Day Tickets to our booking a few months after we made the original booking, and they gave me the 10% and the new balance due, it was very easy to add them on, and no charges for changing the booking at all.
Hope this info helps you save a few pennies, all the more to spend in WDW!
If you want the 10 Day ticket I'm not sure when the special offer price ends or what period the ticket covers, but we go in September and it was OK for that timescale.
Let me know how you get on if you decide to contact them, won't you?