Adding A SFIT to a SPEC - Window Spec

Hello /*,

I need to create window and doors and I don't want to COPY FOREIGN the standard MASTER/STLCATA into my project.

I noticed that the SPEC /Windows-Spec in the SPWL /WALL-FITTINGS-SPWL has SFITs at CATREFs.

How can I add an SFIT to my new Window or Door SPEC?  The error message

I can only add GPART, PRTELE, SCOM, CATE, or SECT!

Thanks for your TIME!