Use a combo box rather than a list box to allow the user to enter values not in the list.
You will need macros to automatically insert addresses associated with listed items, but that creates other issues when using a combo box to allow users to enter a name, which is thus unlikely to be associated with an address. If you want multiple lines, you need a text box control.
There is a lot of useful information on content controls on Greg Maxey's web site - start with the basics at
http://gregmaxey.com/word_tip_pages/..._controls.html then move on to
http://gregmaxey.com/word_tip_pages/...om_events.html