Not possible via the web interface sadly, but you could try this from a script?
:local text ("This is a message I want every one to get");
:local recipient;
:set text ($text . "\n");
:foreach i in=[/tool user-manager user find disabled=no] do={
:set recipient ([/tool user-manager user get $i email]);
:if ($recipient !="") do={
/tool e-mail send body="$text" to=$recipient subject="Your subject";
}
};
If you want to personalise the extraction and body content, you could also extract their First Names (variable is "first-name") and insert that at the start of the email body text.
As it is a script, you can then automate the running of the script based upon some other outage alert.