RDX is not listed in Hardware Compatibility List, so that RDX is not supported so far.
Note that emulation for real library/drive is not supported even if the VTL is listed in HCL. HCL state as follows.
3. The virtual library must be configured with the unique VTL inquiry string as listed. Using the inquiry string to emulate a real library model is not supported even if the real device is supported with NetBackup.
It might work with emulation, but is not supported. Do it your own risk if you want to.
By the way,why don't you consider Deduplication option ?