I think it's posible, because the first .brsar I ever downloaded was already changed for some reason, one of the changes being some characters had their Japanese voices. (Pit, Ike, Wolf, Fox, etc.) So I'm sure it's possible. As for converting it, not sure. I only recently got into adding SFX hacks myself. Also I'm pretty sure GeckoOS won't load SFX hacks, unless there's some update I don't know of. You'll need to convert from GeckoOS to Riivolution. I'd recommend using this video to do that.
Hope this helps.