Have a drop down list add new items

I want a drop down list to add new choices to the drop down in the future.

For example,
I have a column called "staff member" And a drop down list that says "tom, dick, harry". If someone adds Sue to the staff member column I want the drop down to then say "tom, dick, harry, sue". 

Sometimes when I set up a drop down list I don't know all the choices so I want to add then "on the fly". my old database had this feature what you could select from a menu item (no programming) can I do this in SharePoint 2013 online?