Lightning event items should already be storeable in the armoire.

As has been the case with each major and minor patch since launch, the items from the previous event will become storeable at the next patch. E.g., patch 2.1 added all the Lightning items, swimsuits, Halloween items, and Cactuar/Bomb earrings. Patch 2.16 will add the Christmas/New Year's and possibly Valentine's Day items. All items from previous years have always been storeable EXCEPT for the three usagi kabuto and the Santa outfit because those are not Unique/Untradeable.