Welcome to the Community,
you can use vmkfstools to "copy" the RDM to the other storage system (see http://kb.vmware.com/kb/3443266).
Caution: vmkfstools uses the size information in the RDM's descriptor .vmdk file for the copy job. Please ensure the size in the .vmdk file matches the size of the LUN! In case you previously resized/grew the LUN, the information in the .vmdk descriptor file may contain the "old" size, and vmkfstools may not copy all blocks!
André