excel as default program for .dat files

I am trying to set excel as the default program to open .dat files ('choose default apps by file type').  Or conversely, I'm trying to do the same thing by using the 'set defaults by app'.  

When I do the former, the best that Office can tell me is to look in the Store (which I know is not where I need to look).  I don't understand why it doesn't give me a list of all apps on my computer before saying it can't find an app. 

If I go the latter route, it gives me all kinds of file types that Excel can open, but doesn't let me add any new ones.  

Any guidance is appreciated!