If you have signed a contract with your current company you'll need to look over it to ensure there are no conflicts (non-compete, idea ownership, etc.)

If there's no contract or nothing in it specifically preventing outside work then it is perfectly legal.

Your best bet is freelance or short to mid term contract work.

I would start by asking around your network. I'm no social bee, but I've gotten a couple fairly well paid dev contracts this way.

As long as your productivity doesn't drop for your current employer, they likely won't care at all.