You should try to pay as much of the taxes you owe when you file an extension: https://turbotax.intuit.com/irs-tax-extensions/
If you need to combine the remaining balance on your taxes with your current payment plan, you will need to call the IRS and speak to a representative there.
**Say "Thanks" by clicking the thumb icon in a post
**Mark the post that answers your question by clicking on "Mark as Best Answer"