As has been mentioned, a common practice for space teens searching for adventure and excitement and other things a Jedi does not crave was to join the Imperial Academy, acquire the necessary training in space piloting and navigation (learning maneuvers like drifting slowly to the left), firearms use (how to miss Care Bears at close range), discipline and alertness (bitching about drills while a geriatric desert hermit deactivates your space station's tractor beam 30 feet away), intimidation (practicing saying "you Rebel scum!" repeatedly until you get just the right tone and obvious contempt), and how to handle death by crushed windpipe gracefully (in case you're assigned to Lord Vader), and then, afterwards defect to the Rebellion.
The Rebels apparently already had a network established for recruiting disaffected Imperial officers and recruits, since many of them came to the Alliance from Imperial service. Even Han Solo was an Imperial officer at one point, but left to become a smuggler because he rescued Chewie from slavery.