Could the Airbus A350 be converted into the role of air tanker / be used for aerial refuelling?
Yes, it could be done. There is an A330 already working as fuel tanker.


Image Source: Wikipedia

Airbus might build a A350 air re-fueler as mentioned here:


  Airbus is already offering a tanker version of the A330, and could
  (potentially) upgrade their proposal with an A350 air-refueler. The
  A350 would offer a substantial increase in fuel offload from existing
  tankers, as well as the 767.