A different direction, but you could create a small application to read out the excel file and generate the xml file in your own format. Not that hard to do with ado.net. 

But I don't know your requirements, this would make no sence if the end user needs to use this application a few times a day, etc.

ADO.NET read excel
http://davidhayden.com/blog/dave/archive/2006/05/26/2973.aspx

Fill it to a datatable and use DataTable.WriteXml, maby that already gives an acceptable result.