As Mapperz said, there's a difference in the base languages, add-ins for QGIS are written in Python which doesn't have an API for Excel, or office in general. The QGIS program itself is written in C/C++/Python and others. I doubt a request that Microsoft creates a python API is going to be addressed any time soon.
What you can do is export a CSV or DBF from QGIS and then os.system (see this link) to a .net compiled program to do your custom reporting.
Programs can be written in VB or C# and the express versions of these languages will complie to an executable and are free to use for non-commercial applications.