I'm don't believe it's possible to make the system tray icon a drop target, but providing another 'always on top' window to drag things to for import into UR has been requested before and is on the list. Although there are already quite a few ways to get things into UR from other apps:
http://www.kinook.com/UR/Manual/capt...nformation.htm
http://www.kinook.com/Forum/forumdisplay.php?forumid=30