Yes, INTERPOSED is a Scrabble word!

Letter values in Scrabble US: I 1, N 1, T 1, E 1, R 1, P 3, O 1, S 1, E 1, D 2.

Is it a valid word?

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

be or come between
introduce
to insert between other elements
get involved, so as to alter or hinder an action, or through force or threat of force

Also: come in, put in, throw in, step in.