Hi as per my knowledge VSTO has to be a desktop application. However according to my requirement users who supposed to add the data are sitting in 8 different countries. Now I think desktop application cannot be done, as the local data base where the EXCEL data has to be stored cannot be accessible from 8 different countries.
How Can I use VSTO in this scenario?
VSTO can only be used with microsoft office apps, you cannot host it on web. One possible solution for you would be you expose webservice to store data to database wherever it is and your VSTO addin communicate with web service.