I think the best we could hope for is maybe being able to drill into the liquid helium tank and refill it - which is the ultimate determinate of its useful lifetime (that and propellant, but I'm led to believe the liquid helium will run out first).

Probably a moot point though: by the time we can get people out to do such an operation, I suspect a reduction in launch costs might lead to a much cheaper space telescope you could just drop off instead.