You have to edit the ui_character_db.bin to add one more costume slot.
Extract that file from data(us_en)/param/ui/ui_character_db.bin and open it with Parameter.exe
Click an entry in the group. Name 6 holds the value which is a character ID.
In name 7 is the number of costume slots available. Everyone has 8 but Little Mac which has 16.
Locate the ID which corresponds to your desired character and change the value in name 7 to 9.
Save it and put it in Sm4shexplorer.
Build the mod and put it in the SD Card as any normal build.
Parameter.exe? Is that a thing I need to download, then? Could you link me to it?