Never done DLC before, but here’s the documentation: Patching and DLC | Unreal Engine Documentation.
When you cook your project, Unreal Engine 4 (UE4) can divide up your game’s assets into separate chunks that can be independently distributed, such as with DLC and patches. A chunk is a numbered collection of assets recognized by the engine’s asset management system, and when a project cooks, each chunk generates a .pak file which you can then distribute through a content delivery system.
So what it seems like is just:
- Put it in a chunk.
- Cook it into a .pak file.
- Distribute that file to your backers.
If you need to get a list of the assets, you can use the asset registry to scan for and get the assets. This is a twitter thread about that.