Yes, it's possible, but it would be a supremely bad idea to have this NAS exposed to the Internet like that. If you can connect to it, so can any other random person on the Internet. This is one of those cases where you REALLY want to be sure that you think through ALL the consequences. Companies who spend more on just security in a year than you will probably make in 10 years, and have a staff of people who spend all day doing nothing but looking after network security, still manage to get compromised. So you have to ask yourself what hope you would possibly have if those people turned their attention to you. It's just a really, Really, REALLY bad idea once you think it thorough to some of the potential consequences. You can't just stop at, "Oh, this would be nice!"
I live in Singapore, where all the isps here tunnel all their users through a single ip, which is the only ip address that connects to the internet. All traffic is routed through this tunnel, then finds its way onto whichever user that the data is sent for. My question is: Is it possible to access a nas server behind this tunnel via ftp? I know the tunnel ip address and my own ip address behind the tunnel.