Soundtrack Pro (the new version that comes with FCP 5 Studio) can do this …if you have an earlier version, then it probably came with a copy of Bias Peak DV…which can also do this rather nicely.
You can also do what you want using the tools in FCP, though its a bit clunky. To add silence, just add a slug after the edit. Then nest the clip and the slug. Add the reverb filter to the nest.
You dont have to convert wav files to aiff…FCP can read wav files just fine. If you still want to convert, just use Quicktime Pro or iTunes to do the job.