Range expansion is an indirect result of Respiratory Muscle Training.
As the respiratory muscles are strengthened (and subsequently become more coordinated), this allows for the vocalist to explore upper and lower registers more freely through healthier formless. For example, a singer with strong breath support will be able to sing at the extremes of their register with no strain or tension in the neck.
Proper breath support alone doesn't ensure range expansion; however, range expansion cannot take place without proper breath support.