of course. i dont know of a school that would look poorly upon you telling them that you were intersted in them. You could also turn it in to an update letter if anything has changed with your app, if youve started any new activities, LORs, etc. There a few people in my class that have appealed/wrote LORs after being rejected pre interview, then getting interviewed and being waitlisted, then finally getting pulled off the waitlist. Obviously not everyone who writes a letter will get pulled off, but its not going to hurt.