I don't know much about beadlock wheels. Are the strictly for offroad use only?
Answer: You can run them on a DD but most people do not. There is also the never ending debate over DOT approved beadlocks and non-DOT approved beadlocks. I'm sure someone will gladly start that debate...shortly.
Can/Should you run them in a DD?
Answer: In a perfect world I would have two sets of wheels and tires. A set for trail and a set for street. The beadlocks require periodic tightening of the bolts as they do tend to loosen over time.
At what point should I invest in them over a non-beaded wheel?
Answer: That's up to you to decide. However, I would not air down below 10-12 PSI without beadlocks. What tire pressure do you normally run when you're wheeling?
What is the maintenance on them, how often do you need to adjust them?
Answer: You need to frequently re-tighten the beadlock bolts. I check mine before and after every run. Invest in a good torque wrench.
Can you add beadlocks to existing non-beaded wheels?
Answer: Yes. There are several places that sell kits to convert your existing wheels over to beadlocks. Not all wheels will work though. Also, there are companies like Champion Wheel that will do the conversion for you.