I have a balance with a promo rate but another amount with a regular interest rate.
Undebt.it does not yet support multiple, tiered interest rates. However, it's high on my list of things to do. One thing you can do in the meantime is figure out the weighted interest rate of the tiers and use that as your interest rate.
Tiered interest rates are when you have an account with some of the balance at one interest rate and the rest of the balance at another interest rate