Hi Mary, 

Yes, from a food safety standpoint it is fine to freeze your cooked plum jam, but depending on the quantity of sugar and the type of pectin used in your recipe (or lack of added pectin), you might have a jam that is runnier than normal or that "weeps" water. These runnier jams would be safe to use, but you could explore a freezer-specific recipe if you want to have a top quality, firmly jelled product.  Getting jams and jellies to firm up fully is a science! 

Some of the major pectin companies (Ball, Certo, etc.) make pectin products specifically for freezer jams that will reliably produce a good jam. If you use one of these products, make sure to follow the manufacturer's directions because the pectin can break down or fail to set if you do not prepare it in the proper way.

Regardless of what recipe you use, make sure to leave a half inch of head space at the top of your freezer containers for expansion of the jam. The jam be safe indefinitely, but frozen jams are best quality  if consumed within 1 year of preparing them. They will start to lose flavor and color slowly over longer storage periods.

Thanks for using Ask Extension!

Jared