You currently can't harvest magic
I did look up on nearly every pdf i have and the only reference about them harvesting mana is on that entry, everything else talks about absorbing energy to health themselves, or how they are related to vudra deities (look for AP #29 for a complete story, habitat and ecology articles, but most can be seen here). But no, nothing else that can answer this.
So (to me), this sounds like flavor text that was never explored due to limitations on the rules system. Instead, they got a mechanic that allows them to absorb energy damage and heal themselves.
Considering that both the original creature and the region were written back when 3.5 was the default system for Golarion, it is very likely that the authors had Mana Crystals in mind when writing those entries.
Otherwise, you will notice that mana isn't even defined by the rules system.
The closest thing to "harvesting" (storing for later use) are the Spell Absorption spell, pearls of power, and the Rod of Absorption. Noqual golems also seem to be able to absorb magic energy, since they are constructed from magic resistant skymetal.