Hi Pavel,
This option is currently not available in the Kentico 7, but it is a requirement which will be available in the upcoming versions.
There is however a workaround which could be applied to your situation. You will need to use
Global Events which would fire when a new document is created. Now after the document is created you would change the document alias as it is described
here.
Currently there is no better way of achieving this scenario.
Kind regards,
Richard Sustek