How do I create a mult-line custom export scenario with a header and footer in the text file?

I need to create a custom Data Provider and Export Scenario to export a single Purchase Order with all corresponding Purchase Lines.  The file needs to have a header and a footer so I have created 3 different schemas (header, lines, footer) in my custom Data Provider (ACH Provider).  Unfortunately, the Export Scenario screen only allows me to select one of these schema objects.  I don’t want the user to have to use 3 different Export Scenarios and manually put the file together.  How can I accomplish this with a single Export Scenario?