You can do this in version 6, but it isn't as easy as in version 7. You have to create a custom form control.
Here is a thread where I explain how to do it
http://devnet.kentico.com/Forums.aspx?forumid=55&threadid=30037Here is some info/suggestions to get you started:
1. Make your custom form control have settings to indicate which fields of your document type are to be used for the Country,State, and City Fields
2. When you create your document type, use this custom control on the Country field and set the State and City fields to not show in the form. Set the settings mentioned above to these field names.
3. In the GetOtherValues method of your custom form control, return the selected values of the dropdown lists assigned to the fields that were configured in the control's settings.
4. Don't forget to write the code that populates and selects the items in the dropdown lists when the user is editing the document.