One way to do this, that is National Electrical Code compliant. Would be to replace the two prong receptacle, with a GFCI receptacle. While this does not provide an effective fault current path, it should prevent electrocution in the event of a fault.
Technically to be code compliant, you'll have to stick a "No Equipment Ground" sticker on the GFCI receptacle. There should be one in the box that the GFCI receptacle comes in.
Those three prong adapters that connect to the faceplate screw, do not provide an effective fault current path. And therefore would not allow a fault to be cleared by the breaker.
The "correct" way to do this, would be to install a code compliant grounding conductor and three prong receptacle. Though since the receptacle is in a garage, GFCI protection is required anyway.
- Even though most modern devices should work with GFCI devices, some manufacturers still produce equipment that may trip the GFCI.