You say the spreadsheet has the 'correct formatting'. Which is correct: 5400 or 54+00?
I'm getting lost with your terminology and methodology:
I thought I'd join it with our current stationing shapefile.... What is the keyfield with which you are joining the excel spreadsheet and the shapefile ?(I hope you mean feature class, shapefiles are so 90's)
but when I import it into ArcMAP... What do you mean by import? Import is a specific method with which to transfer data from one database to another. For example you import Table X into A.gdb from B.gdb. Do you really mean when I add my spreadsheet as a table to an ArcMap session?
If you can clarify what you have and what you are doing, we might be able to help you.
That should just about do it....