There are a couple of was of adding extra seats to an existing performance to increase capacity.
Method One - Adding a new Area (the quickest and easiest method).
Open the Production containing the performances you would like to add seats to then select the Performances button.
Administration > Production > Edit Productions > Performances
When in Performances select the Areas button and create a new area.
This is the same process when setting up a new venue. Select the New button.
Enter you ID, select the Section you want your new Area to be in, give it a description.
Once the Area is created close the window and select the Seat Range button.
Enter your Range this will determine the number of seat you are adding, A 1 to A 10 will create 10 seats, once your Range has been created ensure you set your Seat Type.