Glad to have you in the Community, keith10.
Adding the bill to one of the reminder lists is unavailable. There's another way where you can see the total amount of the bills that are due to be paid. Go to Expense on the left side and select the Vendors tab. Click the Overdue tab to see the list of the pending bills.
Here's a screenshot for additional reference:
I'm going to forward your suggestion to our product developers. We might know that adding the pending bills to the reminder list will be given a chance to be included in the next product release.
Just let me know if you have other questions. I'll be around here to help you figure them out. Take care!