sure you can. You can enter on the day the visa validity expire and stay in Thailand another 90 days. I and plenty others have done just that. Timed well, a METV can get you close to 9 months in total with 2 same day border bounces 😉
according to a interview I saw with a immigration representative, (on RTW4U).
Extension in country:
Same documentation as the visa requirements.
Entry/re-entry:
No documentation asked
But I wouldn’t rely on that, this is Thailand after all. And allot can depend on the mood of the immigration officer. That interview came off as a tourist ad, I’m fairly sure it will be different from immigration office to immigration office. And arrival location for entry
Some do give a F. Like security at condo buildings and people having to sneak in to avoid sleeping in the street….Happens more often than you might think