You keep a book of occult spells, similar to a wizard's spell-book, and can use its spells to supplement your spell repertoire. Add all the spells in your repertoire to this book for free. You can use the Occultism skill to Learn Spells and add them to your spell-book by paying the appropriate cost, similar to a wizard.
During your daily preparations, choose any one spell from your book of occult spells. If that spell is already in your spell repertoire, you can treat it as an additional signature spell that day. If it isn't in your repertoire, treat it as though it were until your next daily preparations.
While most bards are known for certain signature performances and spells, you're always tweaking your available repertoire. When you make your daily preparations, you can change one of your signature spells to a different spell of that level from your repertoire.
You can translate the emotion and power of a composition to other mediums. If your next action is to cast a composition spell, you can use a different kind of performance than usual for the composition to change any of its somatic components to verbal components or vice versa. As usual for composition spells, this changes whether the composition is auditory or visual.