Yes, EXPOSIT is a Scrabble word!

Letter values in Scrabble US: E 1, X 8, P 3, O 1, S 1, I 1, T 1.

Is it a valid word?

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

state
add details, as to an account or idea; clarify the meaning of and discourse in a learned way, usually in writing

Also: set forth, flesh out.