Is it possible to convert a switched light into an unswitched outlet?
Yes, absolutely!.  You can do this by pulling out the switch, and splicing together the two wires that formerly went to the switch.  Then put a blank cover plate over that switch's electrical box.

You should know however, that you don't have to have a constant power outlet for a garage door opener.  You can have a garage door opener on a switched outlet just fine.  And you can use that switch as a lightweight form of security lockout.

I would recommend instead you put a switch guard like this over it, and just keep that switch on forever.  It's less work and easier to undo.  It also would let you cut power to your opener if you wanted to without getting up on a ladder.



Put a descriptive label on it "Garage door, do not turn off" if you want.  

You can also get adapters that give you two outlets and a light socket with a pullchain, into which you can plug your garage door.



This way you can keep the light in place and also avoid having to do any wiring in the ceiling.  However, you should shop around and make sure it can supply the amperage you want.  Use LED bulbs to conserve amperage and stand up to the vibration of the opener better.

If you cant find a double-outlet-plus-pullchain-light adapter (above) that satisfies your amperage need, then consider one of the single-outlet adapters:



Lastly, if none of those do what you like, you should replace the light fixture with one that has its own built in outlet, so that you don't lose the overhead light capability.  This is rated for the full 15A of a normal outlet, and accepts a grounded plug without the cheater adapter you would otherwise need for the light-socket-to-outlet adapters above.



You can get all of these parts at any common home center.  Have fun!