It's possible. According to news release from Thai government, the covid extension is available to tourist visa/visa exempt entries at the discretion of the immigration officer who may require further information/reason etc. Check with your individual immigration office for their requirements
Lali Aleksandrovna. If tested positive but showing no symptoms, you will be required to quarantine, but many insurances will not cover this situation and may cost you tens of thousands of baht in out of pocket expenses. For more detailed info on this, join this group and you will get more accurate info on the quarantine process
š¹š Farangs Stranded Abroad Due to lockdown in Thailand
If entering as a "tourist" you will be required to show onward travel. I'd advise also to ensure your insurance covers you for asymptomatic conditions, as this is where most people are being caught out when testing positive on the day five test test. The quarantine can be quite expensive if paying out of your own pocket.