Fragnets are supported in P6 by using the Methodology Management Module. The Project Architect wizard in P6 helps you select and import pre-defined methodologies as pre-built project plans from the module. Within P6, use the Project Architect to create a new project plan or to add activities and associated information to an existing project. Additional information using Methodology manager can be found in the P6 Project Management Reference Manual.
You can also copy WBS, a group of WBS, an activity or a group of activities from one project to another. Some create template projects for this usage. You can also save lists of activities in an Excel spreadsheet and import them into an existing project. As mentioned above by Authur, you can also use the Methodolgy Management module aka The Project Architect to do this as well. Primavera P6, as you can see, gives you a lot of tools to perform the fragnet function.