When I say "as a background", I'm being sloppy. It may well be possible to set an image as a background to a page*, but I'm more familiar with image handling in a different way.

First, you need an image of the page, from your scanner. Then drag that image file into MS Word and it'll appear on the page. Enlarge/reduce as required, then either:-

1) right click on it and select "format Picture"

2) double click on it.

Then select the "Layout" tab and click "behind Text". It's then a background image that you can type text in front of...

* I checked :_

"Format"
"Background" - "Fill Effects"

Click the "picture" tab...

It fills the space with the image and tiles it, so the effect may not be exactly what you want, but if you scanned a page-sized document, it should fit to page...