Drilling a steel bolt in an aluminum hole is not gonna be an easy thing to do. Your drill bit is gonna wanna walk off to the side into the softer metal. Then you'll wish't you'd left it alone. DOH!
You will need a nice flat surface on the bolt to start with. Then you will need a way to keep centered and straight.
I've googled for a picture but can't find what I have, but I use a set of drill guides that work pretty good for getting my drill bit centered and helps to keep it straight. This is a hollow guide for the drill bit, and it's inside a threaded nut. At one end, I can get the threads started on the broken bolt if it extrudes past the surface. If the bolt broke below the surface, the other end of the guide is threaded on the outside like a bolt and I can get it started on maybe one or two threads in the bolt hole. I hope you can imagine what I'm describing well enough to think of a way to fabricate what you need. The key to your success in drilling out a steel bolt in aluminum is to drill a pilot all the way through the center. Then keep stepping up in drill size until you get the bolt so thin that you can just pick the threads from the inner wall. Chase it with a tap and you're done.
Have you tried welding a nut over the end of the bolt?