There's nothing in the Slayer metabolism, as far as we know, that would prevent a Slayer from becoming a werewolf, so, yeah, she wouldn't be doing much paroling on full moon nights. Werewolves are apparently very rare in the Buffyverse, though. Throughout 7 seasons of her show and 5 seasons of Angel's, we only saw Oz. his nephew or cousin, the girl from Season4, whose name escapes me right now, Nina, and the werewolf who bit her. Considering the number of vampires and other assorted demons lurking about, they're almost as rare as Suvolte demons.
I would be willing to bet that, at some time in the past, some Slayer did, in fact, get bitten, and standard Watcher policy would have been to kill her before the first transformation, in order to have an uninfected girl on the job. They seemed to consider Slayers as generally a disposable commodity anyhow.