Manually add a centerline

On the ribbon, click Annotate tab Symbols panel Centerline .
In the graphics window, click a feature to start the centerline.
Click a second feature to add the centerline.
Continue selecting features until all desired features are added.

Note: If the features form a circular pattern, a circular centerline is created.
Right-click, and then select Create to accept the displayed centerline.

The operation remains active so that you can add other centerlines. Click [ESC] to finish the operation.