There is no built-in conversion in PI. The function module for such conversion is CONVERSION_EXIT_ISOLA_INPUT in the ECC system.
One approach is to map the source two-character field to a different field which can store 2 characters. Then in ECC, add a logic (via a wrapper or an exit) to translate the 2 char into a single char via the above FM, and then populate it back into SPRSL.