Can I extend my 90-day visa in Thailand without leaving the country?

and just like that we're done.

No

You don't have a 90 day visa! U have a 60 day visa and I assume a 30 day extension of stay

Okay You did NOT get stamped into the country for 90 days on a tourist visa. A tourist visa is VALID for 90 days from the date you got it <- meaning you have that long to ENTER thailand when you'll get stamped in for 60 days. You can extend that 60 days by 30 more days at the immigration office for 1900baht (that would give you almost 90 days) BUT there is NO extension available after that. There is a 7 day extension denied stamp that gives you a week to get out of the country for 1900baht

Depending on your nationality; I would suggest you just go to the border, stamp out of thailand, stamp in&out of another country then stamp back into thailand <- which would give you a 30 day visa exempt entry stamp. Then you're here for the time you need to be.

Maybe this helps

Not that I know of...

You messaged me with this exact question and I answered that message with the same two questions I just asked on here :/

and what type of 90 day VISA do you have now and what is the reason you have it?