for change .igs file to .catpart your needed first change .igs to .step
You should be to join surface igs format and then change to step file
now change step to catpart
"for change .igs to CATPart your needed first change .igs to .step" - rubbish, just open the IGES file in CATIA to create a CATPart
As soon as you import the IGES or STEP, it is a CATPart. It's not possible to edit the actual IGES or STEP. You edit the CATPart. IGES and STEP files are isolated, or dumb. You can reverse engineer them, or modify the isolated elements.