Use Case: Allowing deployer to customize part of a service template by adding a service / node (e.g. attach a monitoring service)

How can a service template deployer customize the template to reference (use) services the provider offers (perhaps as a 3rd party service).

Example:
How can a service template deployer customize a service template to reference (use) services the provider offers (perhaps even a 3rd party service)
such as a monitoring service the provider offers? How can such a service be allowed to attach (method to communicate?) to one or more services/nodes within the existing template?

Notes:
Aaron: High Level Suggestion: Extend TOSCA to support flexible customization.
Matt: Need to distill how this use case does not require some deployment time tooling which would simply create a new service template that would use (add and relate) additional provider's services (as new nodes). Specifically for this use case, would a normative declaration of a monitoring node type help resolve this case?