You can do this with field scripting.  Please see instructions below:

You would create three fields on the master PDF.  You would set them up as follows:
1)  Custom - The placement of this field on the master pdf is irrelevant.  This should be a text field coming from user entry.  On the Data Capture tab, set the user control type to be a Text Area.  On the format tab, set the field usage to hide the field.

2)  DropDown - The placement of this field on the master pdf is irrelevant.  This should be a text field coming from user entry.  On the Data Capture tab, set the user control type to be a Dropdown List.  Set up your locations/addresses in the dropdown list.  On the format tab, set the field usage to hide the field.

3)  Address - This should be placed on the master pdf in the exact location in which you would like the address to appear.  This should be a text field coming from Field scripting.  On the format tab, set the field usage for proof and production.  On the field scripting tab, you would choose to use a VB script.  You would enter the following script:

Ifs(Custom="", Dropdown, Custom)

This means that if the Custom field is blank or empty, then use the contents of the Dropdown field, otherwise use the contents of the Custom field.