How to add task list (set of tasks) to an existing project

How to add task list (set of tasks) to an existing project

When we get an order, we create a project from a template.  Sometimes in the course of working on the order for the customer they want a change, so we need to add a set of tasks around that change to our project task list.  It is always the same set of tasks, so I'd like to be able to just import a task list to that project.

Is there a way to do this?  I have created a task list with the set of tasks needed for the change order, but I can't figure out how to add them all to an existing project