Hi,
There is a FM CONVERT_IBAN_2_BANK_ACCOUNT for converting IBAN to bank account but be careful. In some countries like Luxembourg, when a customer asks for a new bank account, the bank will create a new IBAN but no other bank account. The IBAN itself is used as bank account. In SAP, you will find a value in bank account field, but this value may be only used inside SAP.
Keep also in mind that in SAP there is a functionality "Activate IBAN without Bank Account Number" and if you build a solution now, maybe there is a problem after activating this functionality.