You can use Opt > Preferences > RealDrums to have BIAB always substitute RD for MIDI drums if that's the way you'd like to work.
By default, this only works for PGMusic created styles. If you want other styles (such as Norton's or those found on the web or those you create yourself) to have auto-substitution, if I remember correctly, you need to edit the "MySubs.ds" file in the BIAB folder.
Substitution control for PGMusic styles are found in "a_pgmusic.ds".
The note in the user manual says:
Technical note: The text file a_pgmusic.ds provided by PG Music controls this, and users can make other files MySubs.ds if they make their own RealDrums styles.