To bring up support for a new cloud provider, you would need to build a Pulumi Resource Provider.
This is a gRPC interface, and can be implemented directly, or, if there is an existing Terraform Resource Provider for the cloud, can be implemented using a Terraform Bridge.
For an example of the former, see https://github.com/pulumi/pulumi-kubernetes. For an example of the latter, see https://github.com/pulumi/pulumi-aws/blob/master/resources.go.
If you are up for watching a video - the "How Pulumi Works" video covers details of many of the components related to this - and would be a good place to start to get a broad view before diving in:
https://www.youtube.com/watch?v=YMAe59BYzm4▾
We'll be adding more documentation here in coming days/weeks!