There were no modding tools released for DD, so the only thing anyone has added to the game was a font enlargement patch. Multi-player support would have to be built into the game from the start; I don't think it would be possible to add it in even if there were support for mods.
Divinity 2 was originally going to support multi-player in some form, but during development that put too many restrictions on quest and story design, so it was dropped.
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