Hey there Adventurer,
welcome to modding DFD. As we ourselves are avid Gamer, we really want to support modding of our Dungeon Full Dive going forward!
For now, however, the official modding support is limited to configuring files & adding custom 5e content, no import of custom assets etc yet.
This includes Ability Scores, Classes, Damage Types, Equipment, Features, Levels, Magic Schools, Monsters, Skills, Species, Spells, Subclasses, Subspecies & Traits. (However, as of now, Languages, Proficiencies & Rule Sections are not integrated into the game yet and any added content will therefore not be visible to you.)
For implementing 5e content in DFD and make it easily adjustable, we are using the dnd5e Json data scheme. This means, the json files can be customized how much you want as long as you stick to the format.
You can find the official tutorial of the scheme here: https://brianwendt.github.io/dnd5e_json_schema/
For the OGL content, we are using the Open Source project 5e-database and their json data as basis. Thanks to all of those contributors for their amazing work. You can find that project here: https://github.com/5e-bits/5e-database/tree/main
*%installpath%\\DungeonFullDive_Data\\StreamingAssets\\Rulesets\\DnD5E*