This would be hard to implement as it would depend on who your SMS Gateway provider is and how they would want you to interact with their systems.
Many allow you to send an email to an email address they provide which they will then forward as an SMS Text message to a mobile number as well as to another email address.
This will allow you to use the present system. Your members would enter the email address given to them by their SMS provider so that notifications can be sent to their SMS provider to send to them as text messages.