I have an installment plan for my 2017 Federal and State taxes. Will they add what I owe for 2018 to the installment plan I already have?

[Opus 17]

No. They will expect you to pay this year's taxes in full.  If you owe again, you need to request a new payment plan for this year.  You also need to change your withholding or start making estimated payments for 2019 because if you owe more than you can pay every year, the IRS can tack on additional penalties.