It's so hard, that I've never seen it done. It might work for mazdas, because their hubs use bolts not nuts. But the offsets will change some 10 mm at least. This is 20mm to the track, this changes the forces put on the hub and bearings, which can only adversely affect their load capacity.

Spacers are not specifically illegal, but if a cop doesn't like the look of them then he'll defect you for them.

Unless you can get the hubs drilled out, "Let it go", I know these wheels are probably nice, but they are no good to you.