Using Elderberries in Cooking
One of the very English methods for rhubarb cake is to cook the rhubarb in apple juice first.
Since rhubarb is incredibly tart, the cooking and the apple juice help relax it a little.

The same should work for you with the elderberries. Naturally, you can substitute a sugar solution for the apple juice, but you should dissolve the sugar in the water first and then add the berries.