Hubble's in a lower inclination orbit than Soyuz launches - I doubt it could actually make it to Hubble at all.

Not only that - but you need multiple people onboard the shuttle to manage an EVA - one to pilot the spacecraft, one to operate the arm.

The Soyuz re-entry capsule MIGHT fit in the shuttle payload bay - but on it's own it's no use - you have to have the service module ( the bottom of the three segments ) so you can use its engines to deorbit. - and add the entry capsule and the service module together and you're getting the shuttle bay quite full - with little scope for the hubble fixture at the far end, and even less room for any replacement parts to go into hubble, and space to stow the parts for return to earth.

Furthermore - it would require the purchase of a soyuz - and I believe US trade laws make such a purchase impossible.

Doug