A boss can do anything he wants to do--that's why he's the boss.
And, one of the things a boss can do is to "make" someone, whether or not that person actually killed anyone. Supposedly, there's a "rule" that says that an associate can't become a "made" guy until he kills on the boss's orders. Like all Mafia "rules," it isn't binding.
So, let's say a guy gets "made" without killing anyone, because the boss likes him, or he did some other favors for the boss, or he was a good earner--whatever. If that guy's really clever, he can rise in the family hierarchy. Then he can become the Don by force of personality, by guile, or by ordering others to whack his rivals.