We just recently bought a 26bh. Your layout looks similar to ours. Personally I couldn't imagine adding a bunk bed at that location from a functionality standpoint. Is it possible? Probably, anything usually is with enough time, money and engineering. I'm just not sure it's worth it for the small value it adds.
I know you said you weren't considering an upgrade, but if you did, here is a layout of the 26bh. It has bunk beds in it and would solve your problem. We took our 20yr old daughter with us camping this past weekend, and the bunk bed setup was perfect for her. While no one was above her, she used the larger double bed below to sleep in and had her own space separate from ours.
FYI, someone mentioned a tent, and I honestly don't think that's a bad idea. I keep our old tents in the TT in case we have unexpected visitors that wants to stay and we can't accommodate them. I've had friends set tents up for their kids for years. Most kids like the independence and find it cool to have their own space.