Short extensions would be required at Prestbury and Congleton (down side, towards Manchester) and also Longport (if deemed financially worth it).
The most likely option at Stoke would be to terminate in platform 1, run into the sidings beyond the station and run back out from either platform 1 or 2 depending on pathing which could all be done on existing infrastructure. This couldn't be done until there is a major WCML timetable recast, although could be an option for some peak services.
Extending the current platform 3 would be very expensive as the cross over from the up line would also have to be moved to allow access to it from the up line as it's currently too close, because of existing infrastructure and the fact it couldn't be put on either the road or canal bridge just North of the station for structural reasons, it would have to be moved North to around Sheldon New Rd. This in turn would require a lot of new signalling and make the down line a lot less efficient.
Another expensive, but much less expensive option would be to realign platform 3 so the buffers are in the car park just before the existing exit. This would also require the points to access the platform to be moved North about 20m but would not require the crossover to be moved.
One other obstacle is the platform sighting at Cheadle Hulme platform 3 towards Manchester as it would not be possible to safely dispatch 6 cars. If the guard is in the middle of the train they could only see 2 carriages either way maximum due to curvature. There would need to be a camera system or dispatcher.
As it stands you can run 6 car 323s from Manchester to Stoke but not Stoke to Manchester (not all stops anyway).