Obviously, BIAB doesn't have real instruments for the melody. BIAB will import MIDI, so that's worth a try. And if you use MIDI recordings, a good synth will play them back in a close-to-original sound. The better the synth, the better the sound. There are some great instruments in the Kontakt format (Samplemodeling, Chris Hein, etc.), but I don't think they are a good option at all for BIAB. They are a better good option for DAWs, provided you know a little about MIDI techniques, like CC, keyswitches, and aftertouch. Usually, a simple GM module is preferred for BIAB. A lot of people swear by the Ketron sound module (for sale at PG Music), which is very easy to use and doesn't require tweaking to get a good sound.