The main reason that Farm Trac can't load these USGS files is that the files actually have GPS coordinates built into them and Farm Trac is not set up to handle GPS coordinates.
You may wish to reconsider purchasing Farm Site. Even though you may not be ready for GPS mapping there are a number of other features that Farm Site offers that could benefit you.
In addition to the roads & waterways the unlimited number of layers that you can set up can be used for keeping basic maps such as maps of what crops were in each field each year. You can also start mapping soil types using scanned soil type maps. Often times you can start mapping out different soil test results.
You may wish to consider getting Farm Site. Even if you don't think you'll take advantage of all the features right away, you can start picking up on them little by little.