Hi:
I believe that the only ways you can ADD a worksheet to an existing workbook are to use the LIBNAME engine or PROC EXPORT. If you use ODS TAGSETS.EXCELXP, ODS CSV, or ODS HTML, the entire file (what you put in the FILE= option of ODS) is overwritten with the new report information.
One approach, then is to recreate the entire workbook -- both the existing worksheet named Report (assuming you created it with SAS) and the new worksheet. If you were using BY group processing, this would be as easy as adding BY BYVARIABLE to your procedure syntax and then each bygroup would get a separate worksheet.
Otherwise, I have heard some folks read the first sheet into SAS and then write it back out along with any new information to the same named workbook.
Some useful papers that talk about using TAGSETS.EXCELXP:
http://support.sas.com/rnd/papers/sgf07/sgf2007-excel.pdf
http://www2.sas.com/proceedings/forum2008/192-2008.pdf
cynthia