Yes, DOORKNOB is a Scrabble word!

Letter values in Scrabble US: D 2, O 1, O 1, R 1, K 5, N 1, O 1, B 3.

Is it a valid word?

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

a knob used to release the catch when opening a door (often called `doorhandle' in Great Britain)

Not playable in Scrabble US: DOORHANDLE.