Yes you can achieve your requirement using ajax ModalPopupExtender control...for that no need to user seprate address form...follow bellow approach
Suppose in your employee form below fields are there
Address1-Use Text Box with ajax ModalPopupExtender control so when focus is on Address1 seprate mini window open where you can give data & on ok click
contant of that window display in Address1 field ...for that you have to write little bit code of javascript
Address2-Similar as Address1
City-Use Either Drop Down List or Text Box
State-Use Either Drop Down List or Text Box
Country-Use Either Drop Down List or Text Box
Postcode-Use Text Box
for more detail abt ajax modal popup extendar click on below link
http://www.asp.net/ajaxLibrary/AjaxControlToolkitSampleSite/ModalPopup/ModalPopup.aspx[
^]