Hello Melanie!
I've encountered similar issue with one bank, which provided both bank key and bank account of the customer without tags 30 and 31 before the values. I've written small Python program to deal with this issue - the program analyzed the statement and changed the values in the string by adding the tags. This pre-processing took place before upload. I think the same program can be written in SAP, you have to contact ABAP developer and discuss this issue more deeply. Take a look on the OSS note 494777, which contains information on possible development options. I'm not sure, but it seems that user-exit FEB00001 might be useful.
Best regards,
The Wirtschaftsmann