Processing xls format is not possible with standard means of DSTX (at least not with a reasonable amount of work). You could, however, try to save the spreadsheet in XML format and process that. To my knowledge, everything that's possible in xls is also possible in XML. Or you could access the spreadsheet via ODBC (but that will require a fragile Windows infrastructure). Basically it depends if you are going to process the spreadsheets permanently or rather seldomly, i.e. it would be acceptable having additional manual tasks to perform.
Don't know what IDOC format you mean. There are several formats/standards having such an acronym.