Change Default Value of XML sitemap enable/disable flag
You seem to be asking 2 things. 

The default inclusion status for nodes is set on the node type's edit page (admin/structure/types/manage/MYTYPE). The XMLSiteMap settings are stored as xmlsitemap_settings_node_MYTYPE in   keyed array (status key) if you'd rather drush vset the setting.

As for bulk inclusion, the xmlsitemap table stores all items index for inclusion/exclusion in your site map. A drush sqlq command can bulk update these settings.

#Set all XMLSiteMap node entries as included
drush sqlq 'UPDATE xmlsitemap SET `status` = 1 WHERE `type` = "node"'
drush xmlsitemap-rebuild