Well I got bad news. Stages built with stage builder are saved as .bin files and brawl box doesn't open them (yet). There is one way but it will edit ALL the stage builder stages. You need to replace the textures in the STGEDIT_##.pac All your stage builder stages will then contain the new textures. One thing you could do is build the stage in BB with stage builder models. Other then that, no can do.
Thank you very much for the information