You can enter a latitude and longitude right into the Edit Locations dialog. This works great if it's just a few locations.
If you have a list of latitudes and longitudes, I recommend just using data blending to blend those latitudes and longitude on to your data.
If you really want to, you can use custom geocoding to permanently add your locations to Tableau's list on your installation. But when we update locations in future versions of Tableau, you won't get the updates unless you reimport custom geocoding.