You can already create additional cabin layouts by opening the .XML file that are located in C:\Users\Admin\Documents\Aerosoft\Airbus\FuelPlanner\AircraftData.
Right click on the one that you'd like to modify, select open with Notepad, copy the values of the planes that you want to add (all of it, it starts with the "<AircraftConf>" and ends with "</AircraftConf>") and then modify the capacity.
In example, the first one is the standard layout chosen by Aerosoft, the second is one that I added. The red words are the ones that You should modify.
<displayname>A319 - 124 pax</displayname>
<displayname>A319 - TAP - 132 pax</displayname>