On the ribbon, click 3D Sketch tab Draw panel Include Geometry to associatively copy existing model edges, 2D sketch geometry, and imported wireframe data into a 3D sketch.
You can connect new 3D sketch geometry to endpoints of included geometry.
If you edit the 2D geometry, the segments included in the 3D sketch update. Included geometry remains in a 3D sketch even if the original 2D geometry is deleted.
You can break the link between the included geometry and the parent geometry. Right-click the included geometry and select Break Link. After the link is broken, reference geometry remains in the 3D sketch and is no longer associated with the parent geometry.