Hi WebDude1 and welcome to our forum,
Are you trying to figure out where to get the alias or are you saying that you have it but need to know how to populate based on it?
Assuming you have the list of aliases, you need to filter based on the selected last name to set the e-mail alias. This requires that the secondary data source have LastName, FirstName, and EmailAlias at the same level under a repeating group. Once you have the data, filtering it is easy. You can create a rule when the last name field in the main data source is selected to set the e-mail alias in the main data source is set to the value filtered based on the last name. This can all be done without code. Good luck!
Patrick Halstead
Project Manager at Qdabra