There are 2 ways for this.
Way 1 (Requires PokeBank)
Transfer a Male Yamask/Cofagrigus from Generation 5 that knows Hex. Both learn it at level 17. Then, breed it with a Female Snorunt/Glalie/Froslass to produce a Female Snorunt with Hex. Then evolve that Snorunt into a Froslass with a Dawn Stone.
Way 2 (Also requires PokeBank)
Breed a Female Castform with a Male Gastly, Haunter, Gengar, Misdreavus, Shuppet, Banette, Duskull, Dusclops, Drifloom, Drifblim, Dusknoir, Yamask, Cofagrigus, Frillish, Jellicent, Litwick, Lampent or Chandelure that knows Hex.
Then, produce a Male Castform with Hex. Breed that Castform with a Female Snorunt/Glalie/Froslass to produce a Snorunt with Hex. Then evolve that Snorunt into a Froslass with a Dawn Stone.
So, unless you have PokeBank, you cannot get a Froslass with Hex quite yet without trading for it. Sorry.