Got a possible solution for you. It's not a great one, but it could work. I've created a special file http://www.chienworks.com/media/minus24db-offset.wav that you can download. It's 44.1KHz 16 bit mono and consists of half a square wave at -24dB, or in other words, a flat line skewed slightly above center. Place this on the timeline in it's own audio track. Stretch it out as long as whatever you are trying to offset. Inverse phase if necessary, set the volume slider for this track appropriately (you may just have to guess, see what happens, and guess again), mix with the material to be fixed and render to a new audio file. Hopefully that's simple enough. If you need to do a lot of this i'd certainly suggest getting Sound Forge though. It can handle this sort of thing much more precisely and automatically.