The header can include (and they need not be in a text box):

1. REF fields (cross-references) that pick up data entered in the document
(and bookmarked) or otherwise supplied, or

2. Bookmarks or DOCPROPERTY fields populated by the responses in a UserForm
(see http://www.mvps.org/word/FAQs/Userforms/CreateAUserForm.htm).

--
Suzanne S. Barnhill
Microsoft MVP (Word)

including new pages.

In the header, use fields that pick up the values of document properties
that can be stored with the document. For the user name, you'd probably just
want to use the standard {AUTHOR} field, but there is a general
{DOCPROPERTY} field that can pick up any standard or custom property.

To collect the information from the user, again if it is just a user name
you might either be able to rely on the value already stored by the system,
or else make use of the File > Properties dialog - you can set the 'Prompt
for document properties' option (Tools > Options > Save) to pop up the
dialog on save. Otherwise you'll need to write your own UserForm to collect
the details and transfer them to document properties.

including new pages.