I have never done this myself, but I'd say that you first need to understand the basics of networking, because without it, you'll more likely fail to write the game in a way that it will easily work over the net.
However most of the game mechanics etc. can be developed "offline" and doing so might help you separate game logic from networking and thus keeping your code a bit more maintainable.
Also I suggest to not really aim for MMORPG and more think of it as a normal multiplayer online game. Keep in mind that you can only really have a "Massivly"MO when you actually got the masses to play it.