It is definitely possible to combine haumea and nixosModules! Since the initial announcement, I’ve switched my whole configuration to be based on haumea (into one big module).
There are efforts to improve the integration with NixOS modules with the transformers feature. The built-in hoistAttrs and hoistLists transformers provides a way to specify things like imports and config at nodes of the tree, though at the current state you might run into infinite recursions.
Another option is to have multiple directories loaded by haumea, so you wouldn’t need to use imports or config at the nodes, or you can just have one big module like me and not worry about creating your own NixOS modules, which has been working great for me.