WD40 is ok - a slightly better bet would be a proper freeing agent, such as Plus-Gas...and a lot of patience.
Apply, leave for a couple of days to soak in, reapply and leave...and repeat for a couple of weeks.
You might find that the roller will then move, but only so far. The screw will be bound to the roller with gunk and rust but the threaded stub will be able to turn in the touchpiece.
At this point you have to decide whether you're going to try to turn the screw and risk chewing up the slot, or take it to a repairer and have them finish the job off.
The most common technique is to place a well-fitting screwdriver in the screw slot and use it to keep the rod screw stationary while trying to turn the roller with the other hand. This breaks up the gunk/rust, after which point the screw can be removed.
Experience tends to be the key factor here...a repairer will be able to feel whether the roller is going to move, and either carry on or stop and treat with more freeing agent.
A little gentle heat will help, such as that provided by a hair dryer.
(Edit: Or - what Griff said, plus the hairdryer).