If you just want to set the accompaniment volume for when you play a particular song-- i.e., you want to adjust the volume once, before you start to play-- then I believe what you want to do would be accessed through the mixer functions. See pages E-36 through E-43 in the user guide for a description of the mixer and its functions. Note that the accompaniment uses parts A09 through A16, so those are the parts whose settings you'll want to adjust.
But if you want to adjust the accompaniment volume dynamically while you're playing, then I don't know whether you can do that, at least not using just the keyboard itself.