Ok, so being a CM user in the RP for nearly three years, and studying the ins an outs in the anime, I think I can help you.
Before anything can happen, the user has to be in stage one of their curse mark. Without that, you can't do anything at all. Next, you have two options. Either the second stage transformation (complete transformation), or a partial transformation. The difference is the partial transformation only puts a certain body part in stage two, or grows an appendage that can be seen in stage two. An example of this would be Sasukes wings. As long as he is in stage one, he can either go into stage two, or only change one body part, or even grow his wings.
Now, as for the transformation itself, the ninja cannot control what is transformed with their individual curse mark. If you grow a third leg, you'll have a third leg everytime and you cannot grow wings. In the partial transformation, you could grow your leg as well, but not wings.
So in short, yes, but only if it exists ready in your second transformation. If it doesnt, then no.