Foreigners help making mangas ?

I'm not really sure about if there are alot of foreigners are accepted by working in this sort of thing.

Do you mean the so-called "assistant" where you draw backgrounds or paint black spots for the professional artist? Usually all nationalities are accepted. All you have to do is to apply for auditions that are often announced on manga magazines. If you're skillful enough, and if you can communicate with the others in the artists' office (usually being able to speak, read and write Japanese), they'll hire you. The big problem will be whether you can keep up with the tough job or not.