— While she is a goddess, Julia is able to hear people's prayers. These are not necessarily prayers to her specifically, so perhaps one day she overhears one of the boys praying to Castiel.

— Julia enters the Supernatural verse. Different worlds are part of her canon, she's even traveled to one before.
— Perhaps Cas senses her presence during or after her arrival, either as a goddess or when she is a magicless god, and either he or the boys could go to investigate and meet her.