How do you mount a common pulley on a 10mm left hand threaded shaft?

You can't, that's how. Off to the shop with some 1/2" steel rod and a few minutes later we have two new shafts. The lower shaft that originally had the large pulley had a M10 shaft with a right hand thread, so that one is easy. The left hand thread just wasn't going to happen with the time that I have left so desperate times called for desperate action. I drilled a 9mm hole and using the sander's own motor, filing the threads down just a hair made two parts that were just a smidgen shy of fitting. Perfect. Smear a little high-strength epoxy on what remained of the threads and with a hammer the shaft was driven home. We checked the O.D. of the shaft and it didn't bulge a bit. We'll let the epoxy set for a few hours before messing with the pulleys.