Search through this sight for a while. They do what you want to do. It might take a while to find the answer.
**broken link removed**
If you can find the space, drill a hole into the side of the wheel (use a hand drill!) and use a setscrew. Every other solution includes buying or machining a hub. that the shaft slides into that bolts to the wheel.
In your case, it sounds like the shaft is smaller than the bore, so maybe use some material to close that gap...like epoxy fill or wood. If you use epoxy you can use a 8mm rod wrapped in wax paper as a plug to keep the hole clear as the epoxy cures so you can twist it out and insert the motor shaft.
If it's not very high torque then you might be able to get away with all of this and just JB-weld or epoxy the motor shaft right into the wheel but it may not be as durable or as removable as you want.
Honestly, I've always found attaching shafts to random wheels a major PITA. I wish more motors had square shafts and I had a square drill.
If the bore in the wheel is smaller than the shaft, then get a small round file and a small flat file and carefully file away until it fits! Hopefully, it won't have to be exactly concentric.
Depending on what type of plastic it is, you may be able to heat it up locally until it's pliable enough to force the shaft through, position it, then let it cool in place.
If you have a drill press and a 16mm bit, something like this would do the job, as long as you don't require absolute precision:
Drill the wheel center hole out to 16mm, at a depth > 10mm, mark and drill holes for the mounting fasteners, mount the flange to the motor, then the wheel to the flange.