Getting input outside of the Input class

How would I go about getting input from keys that are not necessarily in the Input class? I am working on an input manager that is able to rebind keys at runtime. I have had success with it for the most part except for when I would want to bind a key that is not in the Input class. For example: thumb mouse button. Not every person has a mouse with a thumb button, but I have played games where I can bind a behavior to that button. I am really unsure where to begin getting that input.