Interesting... I have never tried this before, but I had a think about this... and thought that the perhaps the formatting of the columns in your spreadsheet would need to be included in the export, so I found this article about using XML for formatting and use with Excel.
I would think (not tried) that the URL format should also be determined using this technique?
The best way to see what Excel wants as XML, is to create a sample spreadsheet with these kinds of 'objects'. Then export from Excel as XML and open in a text editor. The hyperlink, for example, will have different attributes than a standard text field. The XSLT should have the way to transform the 'Url-in-field' into the desired hyperlink. Caveat: urls in fields with other text will likely not be able to be transformed so easily as the field that is just an URL.
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