DG, not too clear on what you are trying to do. I am assuming that you want to use a circle for the centerline of a bolt pattern. If so,
in a sketch, select the circle tool and draw a circle, while the "circle" command is still active, look in the feature manager and under the "option" tab there will be a box named "For Construction". Select the box (or hold down the "alt" key and press the "c" key) and the circle will turn into reference or construction geometry. This can be accomplished with existing entities as well. Simply select the line or lines you want changed and select the appropriate box in the feature manager.
I hope this is what you a diggin around for. Let us know if the info did not help with your situation.
Best Regards,
Jay