In that case, it's easy to place an absolutely positioned div outside your container.
<div id="container" style="position:relative;">
then add the following line of markup:
<div style="position:absolute; left:758px; top:120px; width:120px;">Content for your sidebar goes here.</div>
You can play around with width, height, padding, etc. to get the size of the div to your design. You'll get a scroll bar if the browser window is less than the combined width of your container and sidebar, but your container will remain centered on wider windows.
Hope this helps.