You would have to set up a shared variable for the running total in the subreport footer:
whileprintingrecords;
shared numbervar subrt := {#yourrt};
Then in the main report, in a section below the one containing the subreport, you can add a formula like this:
whileprintingrecords;
shared numbervar subrt;
{#mainreportrt} + subrt
-LB