To add a block design source that was created outside of the project, and may reside in a repository of block designs, you can use the Add Sources command as you would for any other source. See Adding Design Sources for detailed information.
When an existing block design is added to the current project, the project might target a different Xilinx part than the part originally targeted by the block design. This will result in all of the IP used in the block design being locked, and needing to be updated. Run Tools -report_ip_status to determine the state of the IP imported with the Creating a New Block Design.