Is it possible to greet someone asking "How is doing?" When I was in US, many times I heard a phrase "Hi! How is doing?" or maybe it was a misunderstanding and the phrase was "How are you doing?"
"How is doing?" is definitely not an acceptable greeting in any form of English with which I am familiar. Most likely you've heard an extremely reduced form of "How are you doing?" The levels of reduction go like this:
It's also possible that some people said "How's ya doing?", as the word how's (also sometimes spelled howse) is a variant of how in some dialects.