This has gotta be the coolest first post yet, eh? Welcome to the site. I would guess that adding an axle with all the add'l labor and parts (brakes, fenders, lines, etc) would be pretty costly. You might be better off buying a new trailer if you are that concerned.
I spoke to the guys at Ameratrail just a couple months ago (Scott, I think...based out of Florida). They have an extra E/X22 trailer on their lot, I think it's dual axle. He was willing to dump it for cheap if I moved it off the lot. I think he said something like $1500, but I don't remember how much he said specifically. Shipping was too expensive to Seattle or I would have done it. Do a search on the forum to find his contact info.
Another option to consider -- if load weight is the problem and you're towing with a Toyota, get a set of suspension air bags. Derek and I have done this and it made our trucks handle much better. Though we both have dual axle trailers. Do a search on this as well. Hopefully this helps!
Derek and all -- I have a trailer question. How difficult is it to add fluid to the brake lines on the trailer? Where do you add it? What type of fluid to use? How do you know how much to add? I'm runnin' low, I think...truck didn't want to slow down very well last night.