Yes, the rim can be offset by adjusting the spokes (if you've got clearancve on the other side, and it hasn't already been offset). Not sure how much is allowable.
KTM do a sprocket spacer - it's around 1mm thick, and is the same shape as the hub/sprocket carrier. I can't find the part number, but I have used mine on KTM's and Bergs. Should be ok to use washers, but you'd have to be careful of thickness, or could misalign the chain (also watch your sprocket bolts - they may catch on the swingarm)