Yes, it is possible to mod those open-source video games, as well as many others. Modding is the process of creating or modifying content for a video game, such as new levels, characters, graphics, sounds, or gameplay features. Modding can be done by anyone who has access to the game's source code or data files, and some tools or skills to edit them. Modding can enhance the replay value, variety, and creativity of a game, and also allow players to customize their gaming experience to their preferences. Some mods can even become standalone games themselves, such as Counter-Strike or DOTA.

Some examples of mods for open-source games are:

-	Wesnoth UMC (User-Made Content) - A collection of user-made campaigns, scenarios, factions, and units for Battle for Wesnoth.
-	Millennium A.D. - A mod for 0 A.D. that focuses on the medieval era, featuring civilizations such as the Franks, Anglo-Saxons, Vikings, and Byzantines.
-	Antarctica - A mod for SuperTuxKart that adds a new icy track and a penguin character.
-	OpenRA: Red Alert 2 - A mod for OpenRA that recreates the classic Red Alert 2 game using the OpenRA engine.
-	Freecol - A mod for FreeCiv that is inspired by the Colonization game, where players have to colonize the Americas and declare independence from their European motherland.
-	Nexuiz Classic - A mod for Xonotic that preserves the original gameplay and aesthetics of Nexuiz, the predecessor of Xonotic.
-	OpenGFX/OpenSFX/OpenMSX - Mods for OpenTTD that replace the original graphics, sounds, and music with free and open alternatives.
-	Contingency - A mod for Warzone 2100 that adds new units, structures, research, and balance changes.
-	The Dark Mod: Requiem - A mod for The Dark Mod that is a full-fledged campaign with 13 missions and a coherent story.
-	Cataclysm: Bright Nights - A mod for Cataclysm: Dark Days Ahead that adds more sci-fi elements, such as bionics, robots, lasers, and cybernetics.
-	Red Eclipse: Parkour Edition - A mod for Red Eclipse that adds new movement mechanics and maps based on parkour and freerunning.
-	Cube 2: Tesseract - A mod for Cube 2: Sauerbraten that improves the graphics engine and adds dynamic lighting and shadows.
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