This is not possible. You cannot use the
<#repeat> tag to fill multiple rows and cells. The Excel merge support is more or less limited to replacing cell values.
When the format is XLSX, the merge happens on the raw file. Efficy unzips the XLSX and replaces each tag that it finds in the XML files with the value from the merge context. The modified files are stored and zipped again. With template scripts, you could try to generate the XML that assembles the excel table, but that is no easy job imo.