You can only keep the outfit on that you don't wanna lose and delete all your outfit slots.
Its the best to buy in 1 time multiple TRANSFER outfits so you don't have to lose them when you want more later.
Otherwise, with BEFF method you will not lose all your outfits and each transfer fit can be created with that method as well. (Check BEFF section: atm only available for old gen)
MAIN CHARACTER GOTTA BE IN SLOT 1 (On the left side)