dotronglong which requests do you want to add additional data to ?
included contains data referenced in the relationship(s) of the resources returned in data, so there's no way to put something there that's not referenced.
If you want to add data to the global forum resource, take a look at how the Tags extension adds a relationship to the forum that loads all tags. Or how core loads groups. Or how Mason loads all fields in the same manner. Or how Catch The Fish loads the active rounds. Basically they all add a serializer relationship, but it's not linked to any Eloquent relationship, the data is "hard-coded".
If you want to return a mix of various resources from a new API endpoint, you should be able to use an IndexController and somehow use different serializers for different items. While it would be valid, that's not a typical way of using the JSON:API spec in Flarum.