Can you convert a METV extension to a Non-O visa for a 90-day stay?

You could if the immigration office that serves your area allows that and you have a Thai bank containing 800k baht transfered from overseas. Depending where you are you will need between 15 and 21 days left on your current stamp. Some offices may also require the money is in the bank for 2 months before you apply. Best to go and ask your local DLT office

You apply for a Non-O Visa in country if you meet the financial (and other) requirements.

Yes you can convert to non-O the same as if you were on single entry tourist visa. But you'll need to have between 15-21 days left on your stamp at the time you apply, depending on your immigration office and you'll need to meet all other requirements as well. Make sure you go to your immigration office ahead of time and get their handout for non-O conversion.