Yes, SPECIALIZE is a Scrabble word!

Letter values in Scrabble US: S 1, P 3, E 1, C 3, I 1, A 1, L 1, I 1, Z 10, E 1.

Is it a valid word?

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

become more focus on an area of activity or field of study
be specific about
suit to a special purpose
devote oneself to a special area of work
evolve so as to lead to a new species or develop in a way most suited to the environment

Also: narrow down.