Bambi <3 You're still playing this game!
And why don't they add a slot for it?
Devs control the archive, GM's can only request for slots to be added to it. There's certain costumes on GF EN that's exclusive to our version only, thus will never have archive slots. I'm not sure if the Black Widow Dress is one of them though. If it's not, you can hope for it to be added in the future~