A "natural" way to do this in Office might be as follows:
a. type some text in Word and bookmark it, say using the name "mybm"
b. select it, then in Excel, use Paste Special to insert it as text.
You should see a formula in Excel along the lines of
=Word.Document.12|'path\document.doc'!'!mybm'
perhaps with {} around it
c. type some characters in the middle of the text and verify that the
change appears in Excel
d. now create a form field (I'm assuming a traditional online form
field, not an ActiveX control or a Word 2007 content control)
e. in the field's properties, set the bookmark to mybm
f. protect the form and type some data in, and see if the change is
reflected in the Excel sheet.
All that said, I have never used that technique "for real" and would not
be surprised if it suffered from all kinds of difficulties. But if it
does work, you can probably skip steps (a) to (d) and type the link code
into Excel.
Peter Jamieson
http://tips.pjmsn.me.uk