I have never seen the 'Flt Pln Full' message in the A320. Perhaps it is a limitation of the FMGC training device.
To program for a return, change the secondary to your requirements and then hit 'Secondary Fpl' - Perf - 4 pushes on the 'Next Phase' prompt and fill in the s/w, temp, Qnh and MDA in the Perf Approach page.