Some Googling suggests that the answer is "no", but I might as well ask... Apparently the field is read-only to AppleScript, and if you hack the XML file directly, every "Date Added" will be set to the date at which the XML was re-parsed. (This means: "don't tell me what your guess is unless you've tried it and it works", ok?)
Dear Lazyweb, is there any way to edit the "Date Added" field of tracks in iTunes?
Current Music: Massive Attack -- Inertia Creeps ♬