You either have to do this by creating a detail_b section that is suppressed except when it reaches a certain count (based on the number of records per page), or you need to use the page footer of the main report for your page footer. If you need further help, you need to explain what you need to display in the page footer and whether the display is the same on all pages where the subreport displays or whether you expect it to vary between pages.
-LB