Off topic is right, i thought someone had found the letter I did to the Tipp Star decapitating Michael Lowery for taking TTUG's credit, ah well.
To reply to your question I don't know. What I do know is that IE have been doing a weird mix and match on the DART recently along the lines you suggest and it is leading to problems of basic incompatibility. My guess is no, and we do know that the basic diagram for these trains are 3 and 6 car sets. Don't be alarmed, their capacity is fairly massive, the six car will hold more than a Mk3 set.
We are the passengers