You wouldn't normally want to do this in a query. If you do it in a query, you need to make sure that your "totals" row is distinguishable from all the other rows and will ALWAYS sort to the end.
A better method would be to create the totals on a form or report. If you are using A2010, Access has a new feature that lets you define a totals row at the bottom of a form in datasheet view. Look for the little sigma button in the ribbon.