Yes, ADDED is a Scrabble word!

Letter values in Scrabble US: A 1, D 2, D 2, E 1, D 2.

Is it a valid word?

ADDED is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

make an addition (to); join or combine or unite with others; increase the quality, quantity, size or scope of
state or say further
bestow a quality on
make an addition by combining numbers
determine the sum of
constitute an addition

Also: add together, tot up, sum up, tote up, add up.