I had a lot of Victoria plums last year, so cut them in half, took the stone out, put them on a tray then froze them like that. When they were frozen I put them in a plastic bag and used them when I wanted to. They were lovely, I like them raw on my cereal in the morning. I should think damsons might freeze in a similar way. Try a few and see how you get on.
microwave them until soft, remove stones from the cooked pulp, put in polybag and freeze. you can use the pulp for jam, tart fillings, eat it with yoghurt etc.