How to convert enclosed surface to solid in CATIA?
Small example from my side. As there is no software supporting to your .catpart file. I do have Catia v5 r19
First of all join all the surfaces to which the solid has to be created
Go to Partdesign workbench
Select the surface and select the closed surface from the Surface based features
Then click on the ok button
Hide the surface to see the solid part