No; I think the game would have had to have been designed from the beginning to support multi-player. Divinity 2 was originally going to support multi-player, but Larian dropped that during development since it was putting too many restrictions on the design of the quest system, etc.
Larian has previously stated that they could not release modding tools for the game because of their use of some proprietary tools that they could not redistribute. I don't know if there would be similar restrictions to releasing parts of the code, even if they wanted to do so.
I don't know if anyone has tried to modify any of the game textures. DD is a 2D game, though, so it would be a lot of work changing that much artwork. Beyond Divinity used 3D characters with 2D backgrounds; this allowed more character customization and many more types of weapons and armour (in DD all large shields looked the same, etc), while reducing the file size required (BD was released on 2 CDs, where DD was 3).
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