Can I convert a single entry Non-Immigrant B Visa to a one-year Non-Immigrant B Visa multiple entry from Chaengwatana after registering my company?

NO, you cannot make it a year-long, multi-entry Non-B inside the country.

You can apply for a yearly extension based on employment IF you meet the requirements.

Go out to Chaengwattana and get the hand out that lists the requirements to extend a 90 day single entry Non-B with a yearly extension. IT is a LOT of documents and you need to get the hand out that lists them so you or your accountant can begin to get them together so you can apply.

You can apply for the year extension when you have 30 days or less left on your current 90 day stamp.