Most schools will explicitly forbid it. Even if the school didn't forbid it, it would be unwise. An additional workload of 20 hours would cut into time that could be spent researching or doing classwork. Honestly I would advise against it, but I understand that sometimes the money would be really nice to have.
That being said, is it possible? Yes. But your program will take longer and the missed money that you will have by staying in the program for a year or two extra is a whole lot more than you'll make at a side job.