Yes, you can store them inside manually when you create the movieclip or you can assign a variable to a movieclip during run time without having created the variable previously.
myMovieclip.newVariable = some value;
Thank you for the quick answer!
This is easy than i hoped.
That is because the MovieClip class is a dynamic class to which you can just add properties (variables) in that manner.
But other classes, such as the Sprite class do not allow for that. If you are using such a class you would need to make your own version that extends the original class to add properties. So just be aware.