RealBand for sure, but BIAB does let you record, as you say, "on top" of the existing audio track as an overdub or harmony, and you can do punch-in recording for whatever time range you want, if you ever need that.

(apparently though, according to my Help file, if you overdub you'll only be able to hear your *new* audio while you're actually recording. you can only hear both the new and old blended together after you've pressed stop)...

- Bobby K.