you need to detail these tasks in a manner that allows for assignment of resources and promotion to a project plan
You need to modify the activity plan to accommodate the additional tasks and allow resources to continue to track their work by using timesheets
you need to save a version of the file that allows the contractors to modify your plan prior to publishing it
You need to check resource availability to ascertain how much free time the resources required for the new project will have during the next 3 months