I'm sure what you want to do is possible, but I think a little more information is necessary to give you a specific solution. How is the data going to be retrieved from the portal? Is it stored in a file or a database?
The data is present in the form of excel files on the online portal.
To be able to retrieve data from external web portal you should use Tableau Web Data Connector API. Check this example: http://community.tableau.com/groups/dev-community/blog/2015/04/04/google-spreadsheet-with-tableau-web-data-connector