For all the content that they niether used a third party app to create nor hardcoded into the game there is likely a tool to edit it in existance. That said, Developer tools for creating content are often not fit for use by modders for one reason or another.
If it were possible to workout the format (possibly with some help from Larian), then it might be possible for the community to create some tools* for simple modding such as editing the items in the game. Once such tools exist, all that would be needed it a way to use the mods without overriding the normal files (a simple way to do this would be as a command line parameter that, when present, is used as an alternate search path for loading data files before looking in the normal directory**).
* Tools that can be made without either specialized controls nor the need to parse special files (for game scripts).
** This is just the easy way; while a way to select a mod in the game would be nice if it done this way (to save time for more important things) then it would not be hard for someone to make a launcher that can be used to select a mod.
