It's not entirely without precedent, particularly in wartime. Note the extremely rapid advancement (by today's standards) of many officers in the US military upon the outbreak of World War Two. Additionally, Dr. David Betz of King's College in London uses T.E. Lawrence, who first joined the British Army in 1915 and attained the rank of lieutenant colonel by war's end, as an example of a successful lateral entry into the British military.
Indeed, Lawrence seems to have greatly benefited from not having experience in the British Army; he was free of the terrible preconcieved notions of warfare which plagued many British officers. Lawrence based his guerrilla campaign against the Turks on his in-depth reading of military history, rejecting the conventional military wisdom of the day.
However, we must acknowledge that Lawrence was truly an exceptional case. Lawrence's upbringing, education, and archaeological work prepared him for guerrilla war in Arabia. Although Lawrence lacked many "military" qualities, most notably the spit-and-polish look of a British officer (even when he wasn't dressed in Arab garb), he made up for his shabby appearance in other areas. Lawrence spent a great deal of time before the war excavating ruins in modern-day Jordan and Syria, becoming familiar with much of the territory in which he would later fight. During his travels, Lawrence became a decent shot with his pistol, learned to speak Arabic fluently, and developed such incredible stamina that he could cross the Nefud desert on camel alongside experienced Bedouin tribesmen. Such skills were hardly commonplace in British officer training programs. Indeed, many descriptions of cadet life at Sandhurst place much more emphasis on playing sports than learning languages.
Moreover, Lawrence' skills--language, physical conditioning, exceptional knowledge of military history--would be even more rare among civilians today. In fact, a recent study indicated that over 70% of Americans would be unfit to even enlist as a private in the first place, due to factors such as obesity, medical waivers, mental health, educational requirements, family readiness, or criminal records.
Even if one does find exceptional leaders who pass the basic requirements for entering the military, there's also a few more hurdles to overcome. These leaders must first be trained in their warrior tasks. For example, picking up a Bell 206 pilot off the street doesn't mean he can be thrown into an OH-58D as a fully-rated combat pilot. The basic warrior skills and tactical leadership take years to hone.
Last but not least, commanders at all levels need experience in mastering the ins-and-outs of military bureaucracy in order to be successful. They need to know the things that you won't read in a military history book: how to coordinate with the civilians at Range Control for training resources and ammunition, how to apply for on-post housing, military legal matters, developmental timelines, property book issues, office politics, additional duties, and last but not least, how to write in the Army writing style.
Indeed, the sheer magnitude of bureaucracy generated by a modern military organization would serve as a near-insurmountable goal for a commander, should he or she have a year or less experience. Lawrence could afford to not have as much bureaucratic experience--his Bedouin army had no payrolls, minimal property, no formations or duty rosters, and most importantly, few individuals capable of reading and filling out paperwork in the first place.
While some can transfer into the military from the outside world, let's face it, it's a rare individual that can do so in a rare situation.