if you do more than 5-10mm you have to replace your studs. they wont be long enough no safely retighten your wheels. thats why they are probably illegal where you guys are from. they dont want every moron losing wheels on the road because they tried to add 20mm of spacing to a lug stud thats only 25mm long.
the safest, easiest route would be to leave the stock studs and purchase, as some have said, 15-20mm bolt on hub adapters. they are about 8000yen per pair.
also, be advised, that with a bolt on hub adapter of 15mm or less, your original 5 lugs will probably poke thru then, which is ok if those wheels have 10 holes in them, or recesses for the excess stud length to fit into. if they are 20mm, theyll probably cover all 5 of your stock studs.
anyone whos dicked with these a ton knows what im talking about.