You have a similar function in AX2012. You are limited however to one AX item per vendor relating to an external item number. This is by design. Is meant for external item resolution to AX part number for such things as electronic messaging item resolution. I’ve known of people removing the index to allow multiple parts per vendor. This is fine on the incoming message, but you will need to store the external item for transactions related to your outgoing message.
If your goal is to just identify a cross reference 2012 has a related item section available off the released item screen. This is not tied to any specific functionality that I’m aware of.