You can backup your GDFS with Setool Lite. Use the option to READ your GDFS. Be careful to use the READ option, NOT the WRITE option, because you would ruin your GDFS. This would save your GDFS into a .bin file.
When you flash your MAIN and FS you also need to add some standard customization (finalization) to your phone. This customization/finalization should correspond to the FS you chose. In your case: topsony's Download Section > Custom Files >w810custpack > W810_CDA102555_4_R2A_Generic_MEDITERR. The MEDITERR region is the same in FS and customization.