expand_less You may take third party classes not listed above with GM approval.  However, such classes will be priced as though they're a tier higher than expected until the GM has had sufficient time to review them in play.  So this means the GM will work with you to establish their base tier, and then allow you to test them out as though they were a tier higher than that.  This is to discourage, but not completely disallow, new content that might be poorly designed / balanced.
Once the base tier has been proven in play, the GM will consider listing the class for all players and removing the penalty.  The higher the demand for a third party class / subsystem, the more likely it will be reviewed and incorporated.