Booming Blade is included in the sheets by default, and will be available as long as you include its source. However, it is not available as a Bard spell, because per the official rules, it is only available for Wizards, Sorcerers, and Warlocks.
When setting the spells for a class in the Spell Selection Dialogue, you have three columns, one for the cantrips, one for spells, and one for "Extra". It is this 'Extra' column that allows you to choose any spell you want and that is the easiest way for you to add an otherwise unavailable cantrip to a class' list of known spells.
You can also add your own custom spells/races/classes/feats/items/weapons/etc. to the sheet, and even change things currently in the sheet, using the "Add Custom Script" functionality. Using the "Add Custom Script" bookmark you can open a dialogue with buttons linking to various resources.