What most people do is rout out a recess for the stepper to mount in so you've got only maybe 5mm of material where the stepper sits (see here). That way you generally get enough shaft to mount a pulley on directly without needing an extension.

I don't think 10mm of movement will be enough for tensioning, especially not on a long belt. I'd go for more like 25 or 30mm if you can.