Hi,
Have you tried to search and read SAP help at all? There is a section explaining how the batch upload/download functionality works in the UME. It is pretty straight forward in the SAP help documentation, plus the answers you have already been given.
As an aside, I would question the desire to have '000's of users created directly in the Portal UME, as memory is telling me this isn't an advised approach. It is often better to link your Portal to another user management solution either through LDAP, ABAP or something else. You may want to research around this too, by reading more of the SAP help content around the Portal UME.
Cheers,
G.