it's a bit late but it can be useful to someone else:
just copy this code in an applescript and create a FolderAction with it.
-- This script will open all nzb files added to a folder.
on adding folder items to this_folder after receiving these_items
--if there are many files added we need to loop
repeat with anItem in these_items
-- we don't want to open all the files, only the nzb
if name extension of anItem = "nzb" then
-- we open the file with the default associate programs.
tell application "Finder" to open these_items
end if
end repeat
end adding folder items to
Hope it will be useful.