ASP.net Dropdownlist - Obtaining values from a different source to the destination

Hi,
I've read countless articles on how Drop Down Lists (DDL's) work but none seem to help with what I am trying to do.
I have a form where I have a DDL which needs to be populated with lookup values from table 1. Once the user selects a value from the DDL (from table 1), I want to be able to write that selection to table 2, along with a load of other things. As an example, if we have a lookup table called tblDepartment like so:

DepartmentID int, Primary Key Name varchar(200)
which I use to populate my DDL, and then I have a form linked to a table containing employee details called tblEmployees like so:

EmployeeID int, Primary Key Depa ...