reversing rims

[NoHachi]

You swap halves...make the inner half the outer half and vice versa. Then just re-assemble. ++offset becomes -- offset.. Thats all there is to it.
You could get bigger problems with getting the 3 piece rims to seal again after reassembly. Some rims are notoriously difficult to get airtight again (keizers for example).

PS the halves of the rim are also sold seperately (see eg ebay.de) so you can construct your own rim, ET, width etc. As far as I know, most of the JDM rims centers use the same boltpattern as BBS..though you might want to check before ordering.