mirc doesnt have any "all connections" message commands such as that, amsg/ame just send to all the channels. besides, what are the chances the same user is sharing the exact same networks/servers with you? you could easy make an alias for it.
...To carry on from above, if you run a search several people have made aliases for this type of thing, it's been requested a few times. Make sure you expand to 'All Forums' and 'All Posts' when you do a search.