The imported MIDI file will not change what it is playing unless if you edit the individual notes in either the PianoRoll or Notation windows.

For your other backing tracks that Band-in-a-Box generates, you can choose if the chords are in Root, Close, or Open voicing. Right click a bar in the Chords view, and select the Chord Builder. You'll see a dropdown menu for this on the top right hand side.

Attached Files (Click to download or enlarge) (Only available when you are logged in)
chord_builder_voicing.png (19.75 KB, 21 downloads)

Best,
Chantelle