Disclaimer: I'm probably not all that qualified to tell you. But as a foreigner trying to do manga here, I guess I can think up an okay answer. 

Think about it.

Is it easy for a Japanese person in Japan to do manga?

No, no, no....a thousand times no.

So that brings us to the original question,  which has the added handicaps of Japanese not being your native language and you needing a visa to even reside in the country. 

To make things worse, publishing companies are having a hell of a time these days with the prevalence of scanlations and pirated copies of manga...So this is kind of not the best time to try to get into manga. Maybe the late 80s would have been the best time to try (too bad I was a fetus for a lot of that time). Anyway, unless you happen to have a time machine, nobody can do anything about that. 

I only know one professional manga-ka (Japanese). I met him in college, he's a nice guy but we only ever talked a bit and now he's married and lives kinda far away. And no, he's not really famous, I don't think any of you guys would know his work ^^;) My other friend (also Japanese) came up to the Tokyo area to be his assistant. She debuted in a magazine, but she hasn't gotten as far as getting a serialization yet. She is now a NEET (meaning she isn't in school, nor does she have a job) living at her parents' house and drawing manga. She is insanely good, but her style is totally shojo manga circa 1970, and that, in the eyes of the publishers, is a huge problem. Pretty much manga is freelance work. The pro guy I mentioned above has his serialization ending soon, so he is currently developing something new to pitch for a possible serialization.

I am now a 2nd year student at a design vocational school called Tokyo Design Academy. I'm in their manga department. Our teachers are brutally honest with us. During orientation they repeatedly say, " Out of the 30 of you, maybe 3 will debut in the next couple of years, and maybe one might get a serialization. The rest of you will need to somehow keep working at it. And if you can manage doing that, maybe you'll make it."

By the way, I am the only non Asian student in the class. They aren't addressing this to me. 

My current goal is to just keep working at it to the level where a magazine debut is even imaginable. In the meantime, I gotta start jobhunting for a fulltime job...

EDIT: I saw this showing up on the main page and decided to add a manga related anecdote. The other day I was meeting up with this woman who said she was friends with the head assistant for Naruto. I almost peed my pants. Then she mentioned that he was so fed up with the low wage and the crazy hours that he actually quit and moved to Canada. He is now an art teacher. It's a hard world ): And now I have no in....