How to edit a table field of which refrence table isn't the same table?

I want to edit a field 'balance' of type 'curr' using format "write balance to ... currency ...". But the reference table is table "kblpw" (Document item amounts in different currencies). Furthermore, there's no data in table "kblpw". How can I get the reference field of type 'cuky'?

Please refer to attaching file "curr.PNG".