Good luck with that. "Hey doc, I'm abusing schedule III drugs right now and want to stay on. Think you can write me a script so I can make this legit? Just thought you'd want to help a brother out!"
Uh no. Pretty sure they'd be more than happy to show you the way out or tell you that you need to provide baseline blood results, which means going COLD TURKEY for at least a month, if not more. There a reason why you want to commit yourself to injecting for the rest of your life?