If you are using MIDI styles and not RT styles you can do this by exporting your song into a sequencer like Power Tracks Pro -- you could also do this in Real Band.

There are two continuous controllers that regulate volume in MIDI, cc7 and cc11.

cc7 is generally used to set the track volume. It's like the master volume on an amplifier. When you adjust the "Volume" in the main BiaB window, you are adjusting cc7

cc11 is called expression and it is used to control intermittent changes of volume of any note or group of notes anywhere in the track.

To create a swell you would gradually increase (to get louder) and decrease (to get softer) the level of cc11.

BTW, a list of all 128 continuous controllers and what they do can be found here:
http://www.nortonmusic.com/midi_cc.html -- note what it says for cc7 and cc11

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