The provider is maintained on GitHub at https://github.com/terraform-providers/terraform-provider-kubernetes
To add more resources you could compile your own version of the provider and add the necessary go code. A look at the existing source code and the Terraform Plugins documentation should give you an idea how to use the APIs and add custom resources.
Depending on the nature of your resources you can decide later if you want to contribute them back for everyone (if they are of general interest), or if you have to maintain your own fork of the provider (if the extensions are very specific and only work for yourself).