Yes, if you use a shortcut key, it is only for that chord. You can't add to it (so Cjb5 won't work). That being said, you could create your own shortcut key to enter it. So maybe you assign l to maj7b5; then you could just enter Cl, and it will substitute Cmaj7b5.
The instructions from the user manual are:
Add your own chord shortcuts.
Have you found a chord that Band-in-a-Box doesn’t recognize? If so, add it to your chord shortcuts file, and Band-in-a-Box will allow you to type in that chord in the future. This also lets you define chord “shortcuts,” one-letter abbreviations for longer chord names (“J” for “Maj7” etc.). If you find a chord that Band-in-a-Box won’t accept like Csus2, when it expects C2 instead you can enter this on a single line (without the quotes) “Csus2@C2.” Then Band-in-a-Box will enter the chord C2 if you type in Csus 2.
The text file c:\bb\Shortcut.txt allows you to add new chord shortcuts. Note that this file doesn’t ship with Band-in-a-Box (or it would overwrite your file!). The file \bb\pgshortc.txt for shortcuts supplied by PG Music. You can add your own shortcuts in a text file you make yourself and name \bb\shortcut.txt.