Yes, HIJACKER is a Scrabble word!

Letter values in Scrabble US: H 4, I 1, J 8, A 1, C 3, K 5, E 1, R 1.

Is it a valid word?

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

a holdup man who stops a vehicle and steals from it
someone who uses force to take over a vehicle (especially an airplane) in order to reach an alternative destination

Also: road agent.

Not playable in Scrabble US: HIGHJACKER.