The easiest way to solve this would be to craete a duplicate of the table, reverse the sort order then hide it. Add a button next to the current table ('Click to Export') that calls a macro that exports / sends to Excel the newly created hidden table.
You'll be able to find the Macro code in the API Guide that ships with Qlikview - if not simply search on the forum.
You could also create a macro that re-orders the table, exports it then re-orders it back to Asc again.
Hope that helps,
Matt - Visual Analytics Ltd