There are a number of ways to do what you want. The way I would do it is base all of your documents on a common template. In that template, store an AutoText entry with your footer text. Use an AutoText field in your documents' footers to call that entry into the documents.

Before you even start, look into the StyleRef Field which will let you use the same footer but have that footer reflect content in the document pages.

Automated Boilerplate Using Microsoft Word
Templates in Microsoft Word