I had a friend that had tons of figs & she kept giving them to me. I didn't know what to do with all of them so I found a recipe for fig cookies that was delicious. I pulverized my figs in a Magic Bullet & froze them in containers (enough for each batch of cookies) & then just took them out & defrosted when I wanted to make a batch. It worked great.
So, that being said, I looked up & found that there are recipes for plum cookies that you might want to try. This one sounds good & you could probably just dice them up & freeze them like I did the figs.
http://freshisthetaste.com/plum-cookies/