It's not currently possible. You'll find that the DEM files that are available by FTP from USGS are at very low resolution, and practically useless for the average farm operation. Looking at the USGS guide to DEMs, it looks like the maximum Root Mean Square Error is one third of the contour interval. Also, of course, the samples in a 1-degree DEM are at 3 arc-seconds, or about 305 feet. This means that every 2-acre square has exactly one sample within it, so the most important details of elevation within an average field will be well below both accuracy requirements of a 1-degree DEM. What this all means is that it would probably be more useful to grid-sample elevations the same way you do soil tests, or to use the elevations your yield monitor already records (the next version of Site supports this directly for most yield data.)
If you want to see your local DEM as a backdrop image, there is software that can convert a DEM to a TIFF image, which you can calibrate and use in Farm Site. It can be gotten from [www.barmy.demon.co.uk] (please don't post HTML in this discussion group.) It's not the most intuitive software in the world, and you might want to run the resulting TIFF through some image processing using Photoshop or a similar program to highlight contour lines, but it can give you an idea of the general usefulness of the DEM data in your area.