727....
The Commercial add-on is quite easy to do...the ATP for a current Biz Jet driver should not be all that difficult if you can remember how to hand fly the machines.
A quick check of Part 61 under the Commerical Helicopter and ATP helicopter sections will lead you to the correct answer. You are correct in thinking it not that big a deal. Read down to the portion of the reg that deals with "additional ratings" and you will see it.