Yes - you can replicate the same source LUN (XtremIO-A with OLD RP Cluster) to mulitple target clusters (in your case the RP cluster with XtremIO-B and the new RP Cluster with XtremIO-C). This can be done by connecting the RP clusters via the WAN. You would connect the RP cluster with XtremIO-C in it to the RP Cluster with XtremIO-A.
The question is - can an XtremIO lun be replicated by 2 different Recoverpoint clusters? I want to eventually migrate hosts to XtremIO C...and fold up old environment.                                          old RP Source XtremIO-A --->RP---------------->RP-->DR XtremIO_B          |                                                                      |         ^                         new RP New XtremIO_C -->RP---------------->RP--> New DR XtremIO_D
Replacing old RP environment