How can I add another module to my existing Terraform deployment plan?
Terraform is idempotent so will only change what needs to be changed.
So after applying your existing plan, if you add more modules or resources and run another plan and apply, it will add the new resources not the whole infrastructure.
Terraform import is used when you want to start managing a resource created outside your Terraform code for example created in the AWS console, AWS CLI).
It basically associates and existing resource with your new Terraform code, so that Terraform doesn't try to recreate it when you run the plan/ apply.