[yupper32]

ISS has airlocks that allow you to leave without removing all the air from the rest of the ship. Vehicles like a Dragon can attach their port to ISS, board, and then perform a space walk through the ISS's airlock.

Hubble is different. It's not like it's a ship that you can board. So you need two things: Ability to attach yourself to Hubble, and ability to leave Dragon to perform a spacewalk. It's not clear whether you can just have everyone in the Dragon suit up and open the hatch. And even then, you still need to attach yourself to Hubble somehow. I think you can via the port... but then you can't leave. Unless you go out the other door? Can you open that from the inside and get out with a space suit?

My rambling isn't meant to be an actual answer. It's more to show that it's wayyyy harder than "Let's just send up some people to Hubble!".

[bluGill]

These problems could be solved. However no current space craft is designed the right way. Maybe it is a trivial modification to Dragon (making it bigger...), maybe it is better to start from scratch. That is a question for domain experts who probably haven't given the idea enough consideration to give a good answer.