I'm unclear what you mean by "building internet subscribers with the SMTP Addresses that already exist within the exchange environement is now allowing th eoriginal account to receive their emails." - that's not the case.
You can create internet subscribers (mail enabled contacts in the directory) with any mail alias you want - you just need to set the SMTP address messages are delivered to. When the mail routers are looking up target destinations, they're using the alias/domain not the SMTP addressed you set for the internet subscriber.
so short story is yeah, you can do this with internet subscribers. If you're interested I have a bulk creation utility for SMTP users (internet subscribers) that can build large numbers of internet subscribers via CSV file (including voice name WAV files if you have them). I don't have it posted on AnswerMonkey since there isn't a lot of demand for such things and TAC doesn't support it but if you want to give it a look, email me directly at "lindborg@cisco.com" and I'll send it your way.