If you are using version 7, then this might help: http://onlinehelp.tableausoftware.com/v7.0/pro/online/en-us/maps_editlocation.html
I think you can manually assign your lat and long to unknown locations via this method.
I currently actualy modify the location information that way but I am looking to have columns feed the coordinate information from a sharepoint list instead of having to edit the dashboard everytime there is an unknown location.
The only way I can think of is to add coordinates to the whole list to remove reliance on Tableau's generated ones.
There are programs and services that do that, e.g. MapInfo, but I have never used those myself.