Dear Manuel
my assumption would be: C14N_SYMBOL_SUBST_REFERENCE is not the right function module to strat with. But the correct one is mentioned if i rmember correct in SAP customizing (creating report symbols)
Use the dummy one of SAP and create a customer specific one; in coding the value retrieved need to be manipulated and then it will work in my opinion and the function modukle need to be aded accordoing to my knowledge at the place you have shown in your picture.
C.B.
