I'm planning to file an extension. Do I have to pay to full estimated tax liability or can I make payment arrangement? I'm currently on a payment plan from other yr.

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.