Way to ignore my question. Don't be stingy with answering questions.
I didn't ignore your question. Or did I? Sorry!
Anyway, your question isn't focused enough. What exactly do you mean by, incapable of joining the military? Physically? Mentally?
People with physical handicaps can join the army too. They can't be fighters; They can still serve as clerks, though, and that's usually what they are assigned for in Israel.
People with severe handicaps are usually dismissed from mandatory service, however, can still volunteer if they wish.
As for mental handicaps, I don't think that people who have been diagnosed with mental illnesses can serve as fighters.
Those who have severe cases of mental illnesses are dismissed.
Harry, so-called "civilian service" is available in Israel, but unlike Germany, it's not an alternative to military service, but rather an option for people who have been dismissed from the military and still want to give away some of their time to the community, or for people who are not recruited by Mandatory law like Orthodox Jews, religious girls, and Israeli Arabs.
I knew this guy who was dismissed from the military after successfully proving that he's a Pacifist. He decided to join a civilian service nevertheless. Now that's a man of values!