You can still download SketchUp 8: http://help.sketchup.com/en/article/60107
Install it alongside the latest Pro version. Then import in SketchUp 8 and save as an .skp file. You’ll be able to open it in Pro 2015 with a warning that if you save it then it will no longer be readable by SketchUp 8.
I’m bummed this is necessary since this was the reason I upgraded to FloorPlanner Pro and I hope they’ll fix the bug soon.