If you wanted RTF, you could just use bordertopstyle, bordertopwidth, bordertopcolor attributes in the PROC REPORT code. But for TAGSETS.EXCELXP, Excel doesn't always use those overrides. Information in the STYLE template gets written to the correct place in the XML generated by the tagset template.
There's a paper by Eric Gebhart that shows altering the interior table lines and rules and borders for TAGSETS.EXCELXP. Here's the link: