Does the 400,000 THB requirement for the Non-O visa extension need to come from an overseas account, or can it be from a Thai bank account?

there is nothing written in the rules about where the 400K baht needs to come from as long as it's in a thai bank account in your name for the 2 months before you apply for the extension of stay . .

The 40K baht can be a notarized letter from your consulate stating you get that much money a month in overseas income OR it can be 3 months of tax statements and pay slips along with your work permit showing you get that in salary here.

Your salary must be minumum 40k and Immigration ask for the tax payments if you use income of a job in Thailand. As far as I know it is 400.000 Thb on a bank account, saved from working here of from overseas does not matter.