@SMK04you can do this by one of your custom fields. I've taken a screenshot for you. I believe of the four custom fields you have, you can only select one to drive the alternate account code. Once you have selected which field to use, you will need to go back to the Account Codes setup screen. There you would see the option to select the custom field, then you can add the alternate codes.
However, big caveat here...the custom field you choose to drive alternate account codes can only have 10 items in the list. So, if you were to select Department, for example, that list of departments can only have 10 items. As I'm looking at your site, you have more than 10 items in all of the custom field lists, so you may need to come up with a workaround.
