I describe add-on templates as subprocesses that don't always happen with primary processes. As an example, we develop educational content for our customers. Sometimes we want these materials to be accredited for continuing education credit, which is a regulated process that includes an application and (hopefully) approval. We will use an add-on template to insert the steps for the CE application into our content development project.
Structurally, I would recommend that your add-on template's first task be a parent. The subsequent tasks would then be children tasks. This can make it easier to dump the add-on as a group.
A custom form on the parent task might help you measure and count the use of your add-on templates.
