Most Haas mills allow you to hook up an their indexer as a auxiliary axis. You still need the box and straight thru RS-232 cable. On the back of the mill there should be 2 RS-232 ports, top for communications bottom for auxiliary axis. By doing this it allows you to program the indexer in your control. You will need to still use the box to set zero, but after that use MDI and your program to drive the indexer. It is just a few parameter changes to the CNC control and box.
I find it a huge benefit to have the program drive the indexer than the box. It makes program restarts a mid-program starts a very easy. No longer do you need to remember to cycle the box thru to the correct point.
Call Mike Grammens applications/service at Haas in California. He knows his stuff.