I think you are right about the teleconverter, and I wouldn't bother with it for now regardless.
My advice is just to get closer. That zoom range (up to 75mm) will get you some good shots from the edge of the field. With these little guys in particular, you can get up close to the side and/or end of the field.
I did spring for a 70/200 zoom (2.8) but pictures close to me are often at 70mm or so. Also, for what its worth, your lense is perfect for basketball or volleyball, indoors or out, assuming you can shoot from close to the court. My 70-200 is often too long, even at 70mm, if I'm shooting my kid's basketball game from the end of the court (under the basket, which is the best spot, IMO).
The pro sports shooters use multiple cameras (at least 2, sometimes 3) with different lenses so they can get long shots or close shots. You won't get every shot throughout the game without having a similar setup. The good news is that you don't need to. Take advantage of the shot opportunities that you get with your existing lens and you should have more than enough shots for a game. If I end up with 3 or 4 shots I like of my kid's game (out of probably 100 frames shot), I'm happy. Any more than that and I don't know what to do with them anyway. Nobody needs 500 shots of a kid kicking a soccer ball, no matter how cute he/she is.
When you do get a longer zoom, try borrowing and/or renting lenses before you spend your money. A f2.8 will get you fast shutter speeds, better low-light shots, and better separation of your subject from the background (less depth of field). An f4.0 is cheaper, lighter, and can get way better shots than the typical consumer f3.5-6.3 zoom. See if there is a Calumet (
http://www.calumetphoto.com/) in your town. They rent pro gear. You can rent a $2,000 70-200 f2.8 for $30/day and shoot as many games as you can stand.
JD