As above if I have a marriage license from Singapore ( I am American, she is Cambodian)

and i am on a retirement extension can she be added to my extension?

Moved to visas.

Immigration will accept marriage certificate issued by a government authority in any country. If it is not in English it must be translated to Thai and the translation certified by the MFA.

As soon as you are married she can apply for an extension based upon being your dependent.

She can not be "added" to your extension!

Your wife may qualify for an extension of stay based on being a dependent of yours.

Thanks Joe :-)

It will be in english

I assume after marriage, she needs to go back to Cambodia to get her single non O visa as cant do it from a tourist visa

She will need to go to Cambodia to get a non-o. Immigration will not do a change to a non immigrant visa based upon qualifying for a dependent extension.