I'd do it in RealBand - as follows. Assuming you have a chord sheet in your BIAB file and you like what you have but just need to add strings. Find a midi BIAB style - or several - with string parts you like. Open the BIAB file in RB. Change the BIAB tracks to ordinary tracks so theydont regenerate Right click a track (easier in RB than messing with different tracks in BIAB) and make it a midi track. Load one of the midi styles you like, right click the empty track and choose generate midi track. Choose strings and you'll have your BIAB track with the string part you like. And as you are in RB you can choose any strings VST you like. And in RB you can try different midi string parts from your favourite midi styles easily and simply.