Hi Volker,
The ConnectX3-pro card is a VPI capable (Virtual Protocol Interconnect) which means, you can configure the physical ports to work with either Infiniband or Ethernet protocols so your first option here is to set one or both ports to work in Ethernet modes.
the second option is to use a bridge device that can translate Infiniband to Ethernet protocols. Mellanox also caries such: you can read more on this page: Products: Mellanox Technologies
I can't really speak for the the Xeon-Phi as i am not an expert of but i recommend you contacting the local sales office and their they have solution architects that can help you define your solution.
I hope it helps..