If their is a combat-capable civilian with previous military experience involved in a situation with the armed forces, would the military necessarily let him participate? My guess would be no, but I didn't want to come up with an alternate scene if this could somehow happen.
I do not believe the military would let the combat capable civilian participate unless the military commander knows the person and is desperate for one more soldier and even then the commander would have to be sure that person would take orders and if a higher up were to find out the commander could find himself/herself in a lot of trouble in spite of how desperate the situation was. The civilian would not have an official rank of any kind which would make any trained soldier hesitate to follow orders from that person and even doubt that the person was still capable of functioning as good as when the person was in the military. Now if the civilian was simply going to participate as a consultant or strategist or something that might be different but as a fighting soldier I doubt it.
The answer is yes. In war-like conflicts, civilians have more "freedom" to act than do the active duty military who are constrained by political limitations. For example, during the Vietnam War, the CIA waged their own covert war against communists because the CIA often ignored borders. They had no problem flying into Cambodia and sometimes they supported clandestine military deep-recon patrols by providing "black" helicopters or suppressive air firepower where US helicopters were politically prohibited. I strongly suspect the CIA works closely with the US military in Afghanistan and Iraq. They probably also worked inside Croatia, Serbia and Bosnia during that conflict. While the CIA is technically a civilian organization, it is a government-sanctioned paramilitary body so you might be tempted to say it is just another form of "military". As far as true civilian soldiers-for-hire, they are employed by independent companies to provide security in war regions. While they do not openly engage in joint combat with US troops, I am certain they "share" information and do not hesitate to back each other up during a crisis. Same way with armed security guards and local police here inside the US. My son carries a gun for a living and he tells lots of stories about how armed security guards and local police "cooperate" in times of crisis or even in exchanging important information to apprehend bad guys.
I'd just write the story and let the chips fall where they may. The advantage of being an author is the fact that you alone can write just about anything you want into a story. You want a civilian fighting? All you have to do is make a situation where they pick up the gun and fight... sometimes "realism" gets a bit carried away...to the point that creativity is stunted..
