It is possible to use two libraries with Dx/Expedition, at least in 2007.8. Point to your local library and install all the parts you want then Package. (Don't worry if you get errors or if it does not complete successfully.) Change the library, install more parts then Package again using "Only extract missing....". This way you get X% from library #1 and (100%-X%) from library #2. Forward annotation thereby becomes a two step process also.