Are search committees often interested in someone who will add to their department's expertise?
Not at all. I'd wager that a majority of hires are replacements to, or additions to, existing interest-groups.

In mathematics, anyway, there is a substantial scientific conservatism, in the sense of caution about change, as well as extreme uneasiness about "new things one is unable to expertly appraise". I guess the good side is that people are wary of being bamboozled, wary of semi-cranks, wary of flash-in-the-pan fads, ... but, also, it creates difficulties in hiring outside already-established expertise.

Another point would be that a hire outside existing areas would not add to any existing area's "turf", so only the altruistic entities would be in favor of it, barring some deal-sweetening arrangement. A similar mild negative would be that it would effectively reduce the weight of pre-existing turf-areas' "vote" in policy and other matters. Who would vote away the strength of their own vote? :)

And it is my observation that this sort of realpolitick would outweigh impulses to "get on the bandwagon", although perhaps not so hugely but that it could still happen.

But, then, if the "new" field is not a "hot new thing", and the given department has no one in that area, I'd bet that there's essentially zero motivation for them to make a hire in that direction. "Can't do everything." ???

So, probably the answer to the question is "no", in most operational situations.