Read and write the Extended registers can be achieved with the use of Extended memory 6X register read and write function blocks i.e. XMRD block for reading and XMWT block for write. These blocks are only programmable in Concept under 984LL not in IEC function block and also depend on the model of the Quantum processor i.e. 140CPU21304 or 140CPU42402.
Documents related to these function blocks are attached for reference.