While it's true that base Essentials doesn't have a ready-to-roll IV modifier for trainers' Pokémon, it DOES have the
Events.onTrainerPartyLoad procedure, which lets you edit a trainer's Pokémon after they're loaded. See script section
PField_EncounterModifiers and add in whatever code you want to make it modify the appropriate IVs for the appropriate Pokémon. There's no need for third party scripts, such as the one Phigo mentioned.
However, note that the way Hidden Power is coded in basic Essentials is not quite the way it works in the official games. It gives the same results as the official games, though...until you add Fairy type to your game (either on your own or by using the Gen VI pack). I think that the intention of coding it differently from normal was so that if people made their own type chart (like that satire game that has over 40 types), that Hidden Power wouldn't be a game-crasher by calling for a type that didn't exist
The most likely reason why GF doesn't have Fairy-type Hidden Powers is consistency. If you change the type calculation to incorporate Fairy, then Pokémon transferred from older games into Gen 6 would be affected by this change and potentially end up with a different HP type. The players who'd spent ages breeding for the perfect Hidden Power wouldn't like this. And since GF introduced Incense items to breed certain Pokémon (i.e. ones that could breed previously but which didn't produce the baby species), clearly they care a lot about consistency.
Fortunately, Essentials has no such concerns about keeping in line with previous Gens, because Essentials has no previous Gens. It gets upgraded, and doesn't worry about interacting with older iterations. Players generally aren't as devoutly attached to their Pokémon in an Essentials game either, because of the complete lack of a competitive scene and, more generally, much less popularity than the GF games. So it's not a concern if the Hidden Power type calculation changes. It doesn't concern me, anyway.
I just thought that Hidden Power should have access to all the types (except Normal and pseudo-types like Shadow and ???). Yes, that's not exactly how the GF games do it, but so what? If I wanted to do it exactly like how GF does it, I would have ONLY allowed the use of the 16 types it could have been, regardless of a user adding in Fairy or Light or Sound or Bird or Banana-type by themselves. If you want to exclude specific types, go right ahead; I didn't want to, so I didn't.