Make a copy of the report that has your 'sum line' in it.
Change the properties to hide details and delete any report/page header sections.
Include the new copy of the report as a subreport in the footer of the 2nd report.
What this does is that it runs the 2nd report, then when it gets to the report footer, it also runs the report with your desired 'sum line' and hides the detail but prints the summary.