There is only one way to get to the nether and it would require a lot of player based coding, it would involve a teleport command that takes you into the nether itself and the only way to get out is to type it in again, sending you back to your previous location.
You can't use a portal on any server unless they have something like this and I haven't seen it anywhere, it's being implemented on the server I play on right now though but I think it's going to be a hard task.