Is there any way to import .xml files into Sonar?  It sure would be a nice feature as I use Finale and Sibelius for notational creation and editing and neither of these programs come close to Sonar's orchestration capacities.

Short answer, no. You can, however, export MIDI files from both Finale and Sibelius and import those files into Sonar. You won't get any of the engraving look and feel (which Sonar doesn't support anyway), but you will get the notes in the right places and generally the matching dynamics. Once you remap those midi tracks to synths/instruments in Sonar, you can play back your scores. The sound is as good as the synths you plug in.