The only way to do it in MM6 is to hire a Gate Master. In MM7, I'm pretty sure that you have to make your way to a land on foot and you have to drink from the central fountain before you activate that particular Town Portal destination. Don't forget! In my first game, I made my way to the Nighon fountain, forgot to drink, and when I portaled out, I had to slog all the way through the tunnels a second time to get back there. I didn't make that mistake again!