I believe you can change a Mod during the game. It certainly works for me when I am testing scenarios and its very helpful as you can see the effects of your changes without restarting
But you can't add mods as far as I am aware to a game you have started. So you'll need the game set up right at the start.
So if you create a largely empty mod - say called 'Cheat' or 'Helper' and enabled it before play there should be some things you can do.
For example you could mod the cost of your top mage down so that they become cheap. Or add some extra magic paths to them. If your being hammered by fire auras you can make some or all of your troops fire resistant. Grant your starting site extra gem, gold or resource income etc etc.
Of course this requires some modding knowledge. But it should all be in the modding pdf. And there are plenty of good modding threads in the scenarios section of the forum on how to do things and what is possible to mod and what isn't. You can then remove the advantages later when you no longer need them.
I am not sure if you can change the map file during the game. But you should be able to give yourself many advantages with just the mod. Of course you could also knack your opponents forces with mods.
I am not aware of any cheat codes that kill enemy forces, give one off boosts of gems or gold etc. But there may be some.