Not for a book! I need to set different margins for odd and even pages but not mirror margins.

Doug beat me to it! That is the solution I was going to recommend as well. Doug says "insert a text box," but note that in Word 2007, using Insert | Text | Text Box, you need to choose Draw Text Box rather than use any of the built-in ones. You can just drag the text box to an approximate size, then fine-tune it later.

Note that the text box may by default will a border, and you'll need to remove that, and it will by default also have a 0.13" "distance from text" on the right side; if this complicates your calculations, you can change it. When you have the text box selected, the contextual Drawing Tools | Format tab will be displayed, and all the tools you need are there, but you can get most of them in a single dialog by clicking the dialog launcher (the small arrow in the bottom right corner) in the Size group. The "distance from text" setting is on the Text Wrapping tab, and you can use the Size and Position tabs to fine-tune the size and position. You may find it easiest to set the size to the height of the paper (11" if this is Letter paper) and the width to 2.6" and the position at Top relative to Page and Left relative to Page.

To remove the border, use the Shape Outline dropdown, selecting No Outline. You can also find many of these same controls in the dialog you get if your right-click on the edge of the text box and choose Format Shape.