If you want one or more rules to apply everywhere, just select the global scope, set the rule(s), then click the padlock to save them. Rules set in the global scope will apply everywhere, on all sites. The padlock will make these rules permanent, they will still be there when you reload the extension.

I wrote a new mini guide for this today (2017-11-27):
https://github.com/gorhill/uMatrix/wiki/How-to-create-rules-which-apply-everywhere,-on-all-web-sites