A Non imm B extension you get from your local immigration and you should take along all the documents needed as provided by your employer - better still, he/she or his representative should go with you. It is that simple (if you have all the right documents). Go along and ask them.
Franck Stylyroper-Bossi Working for free (for a charity for example) is depriving a Thai of work thus it's considered working. Busking and even selling illegal narcotics on the street can get you arrested for no WP - and other charges of course.