How is it possible to determine ABV of a mead which had 1.070 OG before adding 1kg of plums and virtually unchanged OG after adding the plums in a 10L batch?
If you have Excel (or OpenOffice, etc.), the FruitCalc tool will calculate this for you. The reason your OG didn't change (much) at first is that it takes time for the sugars and water in the fruit to homogenize with the mead. But FruitCalc will estimate the effective combined OG, FG, and ABV.