An MMO is written from the ground up to be an MMO. It is probably the single most fundamental design element of the game! You would probably be better served starting from scratch than porting your game.
As to how to make an MMO, that is generally a task well outside the scope of a tutorial. You may be able to find full books on the subject, or possibly tutorials about using an existing MMO engine ( of which there are few for Java ), but a do x then y style tutorial just isn't going to exist.
You should probably start by looking into the various Java.net type libraries on how to handle networking; the underlying logic of an MMO isn't that different from traditional real time client/server architectures, of which there is a wealth of information available.