Is it possible for Cambodian nationals to get a passport in Bkk?

In a word, no.

You can get an emergency travel document that allows you to travel back to Cambodia where you can apply for a passport.

If you were born outside Cambodia to a Cambodian parent you can travel to Cambodia on your foreign passport and obtain a K visa for free upon entering Cambodia. Once there you can apply for the Cambodian passport if you want it.

There is no provision to issue passports from Cambodian embassies, although you can extend existing passports in many circumstances.

Moved to visa for other countries section.

Ok many thanks for the info. Any idea of the cost and time scale getting a Cambodian passport from phnom pehn?

About $300 and 3 days. You can get it as low as $125 if you are willing to wait about 4 weeks.