If you are wanting to add strings to an imported midi file, you would use the .seq (sequencer) app in Biab to set your midi channels of the original midi file. Make sure the channels are set up on the midi file where they don't coincide with the Strings midi channel, which is usually channel 7.

Import midi file, open seq and set channels, select Biab style and mute all but Strings, save as a new midi file.

Trax