u/Aktshisui

I'd like to know how to modify weapons obtained from exotic crates in the main menu. Once during the endgame, I managed to swap my main weapon for one from an exotic crate and then modify it in the menu, but I don't know if it was a bug because I can't do it anymore. Does anyone know how to do it? I wanted to enter with the bazooka as my main weapon and pick up an exotic Sokol from the ground to edit it in the menu.