In a prior version of Word, I was able to create a Text Form Field in a
template file (.dot), give it a Numbering attribute from the Formatting
toolbar (or via the Format function) and then lock the document with the
Protect Form function on the Forms toolbar.

Then, when the user opened the document and pasted lines into the Text form
Field, the lines would automatically be line-numbered.

In Word 2003, only the last line of the pasted text is line-numbered (with a
'1'). Nothing I have tried, including creating a macro, has fixed the
problem.