I just started making custom skins. My son is mad because he plays XBOX while I am on PC and he can't use the skins I make. They definitely need to make a way for XBOX users to upload custom skins. A way to upload via USB as stated by BJTMastermind would be great.
They would have to do it through the one drive app on xbox, that's how you get custom gamerpics and since both minecraft and onedrive are owned by Microsoft it wouldnt be that hard to integrade it into the game