Yes, it is possible. All you have to do is complete all the story quests on a Sith Sorcerer
Sith inquisitor, not sorcerer. so It can be either Sorcerer OR Assassin, despite normal storm being a sorcerer ability.
And im afraid this is the only way I know of. But dont worry, there are other reasons to play one character on each faction, so use this as a bonus! A nice little incentive.