Populate word document with listbox selections

All this makes me wonder why you're using a userform rather than, say content controls or formfields.

As for programmatically creating & using custom document properties, see: http://support.microsoft.com/kb/212618/en-us

A simple demo of populating a custom document property from a userform and updating the document is attached. The custom document property is named 'Demo'. The corresponding DOCPROPERTY field could be positioned/repilcated as often as need be anywhere in the body of the document.