Hi Mike,
I know you must be correct but I'm doing something incorrect because I simply can't do this, well at least not always.

I have loaded a BB RealTracks song that holds my chord progression and style that I am happy with. Now I open this song in RB. Now I want to add some tracks of my own, in addition to those generated by the song in RB.
When I select an empty track and use the Generate command to create midi Bass, for example, I am told that a Midi Bass track was not found in the current style.

I know that if I now load a new style that DOES contain Midi then I wil;l be able to generate, but now I stand to have my existing tracks change (unless I freeze them).

I was hoping to be able to select from a list of styles just the Midi Bass I was looking for, without changing the existing song!