Add this option like new feature in "Page numbers" panel or like parameter in the pattern field; this last option I think it's not possible currently, is it?
Now, it's not possible to add page numbers starting from number different than 1. For example, one document with 3 pages that we want to numbering, but we want that first page be page "8", second "9" and third "10".
One solution could be add this functionality like new feature or implement into pattern field with something like "{NUM}+7".