ReportBuilder's requests and responses do not contain any formatting; it simply sends and receives raw numbers. If you want to format data requests a certain way, you can find this functionality by using the Format Cells feature within Excel:
http://office.microsoft.com/en-us/excel-help/format-cells-using-a-different-locale-HP003089677.aspx
Right-click a cell and select Format Cells. You can customize the decimal format, preference of where commas and periods are used, and so on.
