If you are still learning, I'd suggest fixing the nose wheel. It will save a lot of props when you are learning landings.

If you really want to convert, you need to do two things. 1) move the mains in front of the CG and 2) install a tailwheel bracket and wheel. You can get tailwheels and brackets made from many vendors. There are many different styles but something simple like this

http://www2.towerhobbies.com/cgi-bin...?&I=LXD850&P=0

would work. It would require cutting into the rudder to install the steering shaft.

As for the mains, I would move them in front of the CG as far as they are behind it now, probably about an inch or so.

If possible, I'd suggest finding some one locally who is experienced to help you with the mods, or at the very least, have them check your work.