Yes, you can add additional drivers to your car insurance policy if they are not related to you. If the person is a licensed household member they do not need to be a relative of yours to be placed on your insurance policy as a driver.
If however the person does not live with you than whether they are a relative or not they may not be able to be placed on the auto insurance policy though. It depends upon the guidelines of your insurance company.
Those that live with you and have a driver's license - be it your child, parent, roommate, friend, etc -are normally required by an insurance company to be added to your car insurance as a driver since they have access to your vehicle and thus are likely to drive it. If you have a friend that borrows your car from time to time and want to add them as an occasional driver than contact your insurance agent to see what your provider's guidelines are for an occasional driver and how to add a non-relative to your policy.