How to write your own custom data builder, custom resource provider and custom group schema.

Hi guys,

I'm trying to implement an asset override system using the Addressable system. Basically I'm trying to write my own custom data builder in order to include in the content build only specific groups needed for the current target platform without doing every time a new game build. The problem is that the default build scripts included with the package are internal (so I can't extend them) and some of the classes it uses for the content build are also internal (so I can't use them). Did someone manage to write a custom data builder? I've also wrote a custom group schema inheriting form AddressableAssetGroupSchema to add some info to my groups useful during the build, but I can't add it in the group. Do I have to put it in a specific folder?
An other thing I'd like to do is to support a content DLC without shipping a new game build. For this I was thinking to write a custom resource provider to get an asset from the DLC bundle if present. Is it something doable?

Thank you.