Fsm variables can be found here:
FsmComponent.Fsm.Variables
Each variable type has its own array:
FsmComponent.Fsm.Variables.FloatVariables[]
FsmComponent.Fsm.Variables.IntVariables[]
...
Do you want to add variables at edit time or run time?
Most editor stuff is done by an FsmBuilder class, but this is not documented. You would have to poke around to figure out how it works...
If you're talking runtime, you could make a new variable and add it to the array yourself:
var myFloatVariable = new FsmFloat("myFloatVariable");