Well...you can't exactly create your own...
But there are 2 other "hidden" stances...
First, open the console, and enable cheats with "helpusobi 1"
Then you can type "setforceall 10" or another number 5 or above... This will set all your force powers to the given number, if you don't want to do that, you can try using "setsaberoffense 10" and "setsaberdefense 10", which should only change your saber ability.
What this does, is when you change your stance on heavy, it won't appear to change, the same for light. So it adds a heavy and a light. The heavy is a one handed type heavy... That it would appear Tavion uses in the game... The light stance is a kind of mix between medium and light.
Although, another cool thing with setting your force for over 3, with Mind Trick, you can some times actual control the NPC you use it on. So if you Mind Trick 5 a Stormtrooper, you'll gain control of him for a short while, through your controls, like when you control a robot in the game. The higher your force level, the longer you control him. To the best of my knowledge, you can't get out of it, until time runs out. If the NPC is packing heat, they fire at a much faster rate.