The only way to do this is by adding a map reveal trigger to every individual map. You can either do this via the map editor or by editing the map with a text editor, but take in mind that you won't be able to play online with these maps anymore afterwards (unless the person you're playing with has these same edited maps). Since you'll be editing multiple maps it's quickest to use a text editor.

If you use a text editor, you'll have to verify that [Triggers] isn't already present in the map's code; to do this you simply have to open the search tool by pressing Ctrl+F and search for [Triggers].
If no results are found you can simply proceed to add the following code to the map:

It doesn't really matter where you add the code (I'd put it all the way at the top for convenience), as long as you don't overwrite any existing code.

If the searching for [Triggers] did return a result, you'll first have to check if a map reveal trigger is already present, but disabled.
A disabled map reveal trigger looks like this (the numbers before the = can be anything): =Neutral,<none>,Reveal,1,1,1,1,0
The first 1 after Reveal means the trigger is disabled; simply change that to 0 to enable the trigger. So the line above should become =Neutral,<none>,Reveal,0,1,1,1,0

Finally, if the map does return a result for [Triggers], but there's no map reveal trigger present, you'll have to add the necessary lines under [Events], [Actions], [Tags] and [Triggers] yourself, but make sure that the number before the = is unique and that the numbers that currently correspond still correspond after you change them (so the number before the = under [Triggers] should correspond with the number before the = under [Events], [Actions] and the number at the end of the line under [Tags].