Yes, TRANSOM is a Scrabble word!

Letter values in Scrabble US: T 1, R 1, A 1, N 1, S 1, O 1, M 3.

Is it a valid word?

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

a window above a door that is usually hinged to a horizontal crosspiece over the door
a horizontal crosspiece across a window or separating a door from a window over it

Also: transom window.