If you owe federal taxes for 2014 or prior years and owe again for 2015, you can include the new balance (tax liability) in the current installment agreement. There will only be one installment agreement which includes all of the tax years that you owe. You will want to contact the IRS as soon as possible to have it included because a new balance will automatically default your current agreement (It is one of the terms and conditions.)
You can call them at 1-800-829-7650 although there will be long wait times since it is peak tax season.
You must be current on your existing payment plan and your upcoming years tax must be paid in full, or you plan is in default.
Contact IRS to update your plan before Tax Day to add another year.
(the link given does not include any way to increase your balance outstanding).