There's no way to add maps to Streets&Trips.
You might want to look at the products on DeLorme's site. They have some great map products for topographical use and might have something for the national forests. Take a look at their
Topo USA 8.0. Even if it doesn't have all of the stuff you need on it, there are downloadable maps at DeLorme that you can add that should have it all covered.
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