I would advise you to update C2 to the latest version since there's no good reason not to as far as I can tell (r114 is a stable version). It will not break your existing projects, and if you're worried about that you can just make a backup before updating.
Anyways, you can do what you want of course. If you should need atan2 for anything, here's a way to do it in C2:
It's used in the system angle() expression, which as far as I'm aware, is the only purpose for atan2 - calculating the angle between points.
If you really do need atan2 for something else, I'd be interested to know what it is! You can also get an atan2(y, x) anyway by doing angle(0, 0, x, y).edited><editID>Nimtrix</editID><editDate>2012-12-29 16:13:28</editDate></edited>