@Florian *******
, you can apply for a Non-Immigrant Type visa from a 30 day visa exempt, a 60 day tourist visa or the 30 day extension on either of them.
I'd go to the Chiang Mai immigration ofice and see if you even can do an in country conversion to a 90 day Non-O for that reason. I don't see why you couldn't IF you met the requirements but definitely check with them