there are multiple ways.
one, cut the welds holding the centre in to your old outers, buy new outers of desired width. weld centres in.
two, cut two rims at different widths, and weld together. use four rims to make two.
neither are advised for a diy attempt, unless offroad use.
you can buy new wheel outers for on road use, and they can be done professionally.
-dave
btw, i have some stock ke35-55 steel centres in brand new ROH 13x7 outers, (could be made in any offset desired within reason)