There are .net libabries (even free) out there (google will help) that can read xls and/or xlsx.
For example this one: http://exceldatareader.codeplex.com/
], that can read borth, but no formatting as I see.
Choosing the right one depends on how exactly this export should be performed (only content matters, or you need to follow exact styling), and what exactly this excel file contains (only data and formulas, or are there any external references, or are there even embedded objects).
I think if you have embedded objects, you will not be able not avoid using Excel or some other excel-compatible spreadsheet editor interop - or you will have to code a lot.