Hello Gabriele,
as of DW 7.0 Beta there is no way of creating a multi-archive search globally or via administration. Multi-Archive Searches are always user-created, merging accessible, existing search dialogs for the single archives. You could create search dialogs for the seperate archives pre-filled with the required values, but creating the multi-archive search still has to be done manually for each user.
Greetings from Germany,
Simon H. Hellmann
DocuWare System Consultant