I just extended this morning at the Bangkok immigration office. I arrived on November 30 on a 60-day tourist visa, and did a 30-day extension on that in Chiang Mai (they wouldn't let me do the covid visa until I'd done that extension). I did the 60-day covid visa in Bangkok on January 25, and that was treated as a conversion to a new visa, not an extension of my existing visa, so I only got 60 days from January 25. But today, I went in to do a 60-day extension of that covid visa, and it was treated as an extension -- 60 days added to my current covid visa.

So, not sure how things will look in other offices, but at least in Bangkok, it looks like you should be able to get an extension on your existing covid visa. The one thing I could see being an issue for that is if you have to wait a minimum number of days from your last visa conversion/renewal/extension before you can apply. Like, if you extended on February 28, then maybe it's too soon for you to try to extend on top of that.