How to allow existing users to add a mobile phone number (not on registration)?

Is there any way to use the SMS User submodule to enable users who have already registered an account (before I had SMS Framework enabled) to register a mobile phone number?

In my case, I want to use phone number verification to reward users who submit their number after registration, not as part of registration itself.

Any advice about how to implement this with SMS Framework or another module would be greatly appreciated.