You can always just make a new equipment type and call it something like "Sub-Class". Name the equips after the class they're supposed to add the skills to (So something like "White Magic" for this example), and in their effects have them add the skill type. In my example, I've added the "Monster" skill type to my character:
This character ordinarily only has access to the "Physical", "Magic", and "Lineage" skill types, so it is working.
EDIT: I am assuming that, with this type of system, you are sharing Bravely's system of the skills being categorized by class and not by type a la my system (Physical, Magic, etc.). I am also assuming that you are, unlike me, using levels and classes, though that's not much of an issue if you're using a Skill Learn system. The biggest wrench for this workaround would be if you're using the same kind of skill classification as I am, though I could probably figure out a workaround for that, too.
