I read, some time ago, that people who converted their K701 into dual-entry bought a Right cup to replace the Left one. Then they drilled a hole in the center of each cup.
You can't really buy a Left cup to replace the Right one because the stock cable entry isn't centered. This would place the Right cable entry at the back while the Left one is at the front. Ugly.
But anyways. As I said earlier, converting an AKG phone to dual-entry is overkill. You still get all the advantages of balanced operations even with a single entry. It's a nice project, though.