The focal length of the lens is not modified in and of itself through the use of extension tubes with fixed focal length lenses. In the case of view cameras the same effect is realized by increased bellows extension beyond infinity focus.
Through these alterations (extension tubes and bellows extension beyond infinity focus) lens to film plane distance is increased and the lens to subject distance is decreased which affects (increases) magnification. As magnification is increased exposure must be increased to compensate.
To answer your question about focal length. The focal length of an asymetrical lens would be the distance from the lens nodal point to the film plane when the focus is at infinity.