Numbers ’09 supports Excel native format (.xls) but it can’t convert from an Excel XML Spreadsheet. In fact, if you rename it as “xls” and try to open it (trying to fool Numbers), you’ll probably see all the XML but not the actual spreadsheet. Which is more scary than Psychosis II back in the 80s.
Your only bet is some CVS or TEXT format, which excel can even export with Macintosh format for line endings and carrier return (if I’m not mistaken).
The big drawback is that all format and binary stuff is automagically lost in translation (certain pun intended). ;)
You’ll get the data, but not the format.
Another alternative is trying to automate the Excel XML to Excel Native format, even if your webapp outputs a XML, you could try to “transform them” before anything else.
For that, I haven’t investigated but you can start with a google query like this. There seem to be some Windows solutions, maybe something you can use/try.