The issue is that the road cassette is 1.85 mm wider than a mountain freehub body, right? I'm getting out of my depth here due to my lack of direct experience w/11-speed kit, but some searching on the forums suggests that a SRAM 11-speed hub will fit a Shimano mountain freehub. So you could look at the SRAM PG-1130 in 11-26 gearing, and maybe that would fit within the range of your current derailleur. Another idea if you just want to run a quick test is to set aside one of your 11-speed cogs and use some spacers to run the other 10 cogs of your current cassette and set limit screws to prevent over shifting. That's hack that I would personally have a go at, but I wouldn't blame if you didn't want to go there.

Hopefully someone with more direct experience will chime in.