Indeed you can using Microsoft Visual Basic for Applications Extensibility.
Although I would stick module calls into an addin and hook into the Application/Display event objects so you only have 1 instance of your common code.
It would be a good idea to have an add-in so you only have one copy of the code (as Rhys pointed out).
I can't help but wonder what you are doing in a display that needs to overwrite ThisDisplay object, it would be interesting to hear a generic description of what your code does. (to get an idea of how you are using VBA and get people in context).