Technically speaking you can read any file, from anywhere.
The challenge is parsing the data. There are builtin steps to parse XML documents, and plain text documents (i.e., txt, csv)
Wikipedia: MS Excel File Format
Snippet:
Microsoft Excel up until 2007 version used a proprietary binary file format called Binary Interchange File Format (BIFF) as its primary format.[4] Excel 2007 uses Office Open XML as its primary file format, an XML-based format that followed after a previous XML-based format called "XML Spreadsheet" ("XMLSS"), first introduced in Excel 2002.
So perhaps the newer versions can be read via XML, but I have not personally tried this.
As for reading documents off other servers, hosting them via a web service is always a clean solution:
Document myXMLDoc = URL[http://www.company.com/files/customer/1022345/xml]