We have implemented this by following approach:
1. Use the file attachment control to upload the file.
2. The file gets uploaded the c:program files (x86)K2 smartforms runtimefiles
3. You need to write a expression to get the uploaded file name because K2 generates a GUID after upload.
4. You can have a web service to which you can send the file name, in the web service you can read the excel file and load to the DB.
The above approach will surely possible and it is the faster way to achieve this requirement. We have successfully implemented this solution for one of our client.