User blog comment:Windindi/About Nintendo vs CAPCOM, Demo v0.1/@comment-8622820-20141021042459/@comment-3495717-20141021125602

Ah, I see your problem now. MUGEN 1.0's .sff files cannot be edited in FFU because they're sffv2, not the regular sff.

You will have to use FF3 if you want to alter the screenpack.