To include the bank account number in the direct debit file, check the option to "create credit offset transaction" after going to Tools > Create EFT Transmission File.  This option will include an extra line in the Direct Debit Transmission File that includes the account number selected in Batch.

Note:
The software that financial institutions use to process the Direct Debit Transmission File may  or may not be configured to read the file with the extra line that is created when including the account number.  Therefore, please check with your Financial Institution to see if they will be able to accept the Direct Debit Transmission file with the additional line.

In the meantime, save the Control Report that appears after the Direct Debit Transmission file is created as it lists the account number selected for the batch.

Export the report to PDF format and then submit this to the bank along with the Direct Debit Transmission file. 

Note: 
This option will also need to be discussed with the financial institution prior to implementing to ensure this process is accepted by the institution processing the transactions.