What mapmixer do is:
1. Add empty script to the map. (that is required for MMh55 mod to work)
2. Does whatever settings you choose.

So the answer is yes.

To do that you need to :
1. Extract the default maps from data.pak archive. I believe they can be found in data.pak/Maps folder.

2. take the map folder and make it into a *.h5m file as other ordinary maps.
3. Use mapmixer to patch the maps.
4. Usually the last step will be to put the map in Maps folder but as it already exists I am not sure how to continue here. One way is the extract the .h5m archive and use the files to overwrite the same files in data.pak.

OR

Open the .H5M archive and change the map name (for example change it to "Warlords_patched") and after that put it in game folder /MAPS directory.

In game you will both maps - "Warlods" and "Warlords_patched"


Note: If Mapmixer is used on user created maps that include custom scripts it may wipe the scripts (not tested) and ruin the game experience. What I can advise is taking a copy of the scripts before patching and applying them after that.

I hope that helps!