I have 4 large jars of plums (Santa Rosa), which I bottled. The instructions said sugar would not be needed, however, they are so sour they are awful!!! What can I do with them rather than throw them out? Can I make jam from them now that they are already cooked?
Assuming they were canned safely (sounds like they were acid enough that this is likely) you can certainly dump them in a pot, add sugar (and possibly spices), stir a lot and cook (and stir) more to make jam, then can that if you like. Be sure to stir well until all the sugar is dissolved, so it does not burn.
Making jam is a long cooking process, so "pre-cooking" won't bother it.
I have done this with a different variety of plum when there was a bumper crop one year and I was scrambling to preserve it as fast as possible before it could rot. I then made jam out of some of the quick preserves when there was more time.