ok, i take it you don't have a blowtorch off any description? if not i can think of two possible possible ways.

1. cut out 2 new wheels and cutout the hex and use like that with washers on either side (when i get home i can show you a photo what i mean on my robot)

2. buy some banebot wheels with the required hub. (expensive Plus shipping and possible import tax)

if you have a blow torch you could do something similar what Ellis did in his video here