Yes, you can build a level editor and use that to create maps to load. As far as loading new graphics, that would be a bit trickier, but it's also doable. You can load animation frames to sprites from files at runtime. You could even script things like cutscenes or enemy AI and such with your own custom scripting system, saved as text files.
It would take a lot of prep-work and organization to create entirely new game content without changing the .exe, but it could be done.