Looking at the other thread linked by Paul, you have a bunch more information there that complicates this further. I'm not a member there so I'm not posting there.

Firstly, your docm files all have an 'attached template' so you can still house building blocks in Normal.dotm, the attached template or even an addin. If the docm files already have a common attached template then that would be the simplest place to put your building blocks.

Secondly, are you trying to do all of this via code or do you want to use the Word GUI to link to the xml elements (fields)?

Thirdly, the code shown for Document_New is not achieving anything let alone doing this...
You generally bind a Content Control to an embedded XML Custom part's xml element. You might replace or update the xml custom part but I wouldn't use the terminology of binding it to the document.

__________________
Andrew Lockton
Chrysalis Design, Melbourne Australia