The main thing you can't do is :
Selling openarena without giving its ported source code.
I suppose you can port the source code to any architecture you wish.
Once you've done this, you'll have to distribute explicitely under GPL licence your source code,
modifications included.
I'm not a lawyer nor even a developper, just a simple OA player, but you should get a good understanding on what GPL and free softwares are.
* Cacatoes didn't refresh the topic for new answers before posting :-]