If you live in a state that recognizes common law marriages, you may be able to qualify for a spouse rider. However it must be noted that only 13 states legally recognize common law marriages, contrary to common belief. If you do not live in a common law state or if the state you live in does not recognize your partnership, you may not be able to get a spouse rider.
Some insurance companies have recognized the importance of offering domestic partnership policies, even if the state they are in does not recognize common law marriage. In this event, the insurance company may ask you to prove specific things about your partnership such as joint assets, proof of living arrangements or shared banking accounts.
As of this writing, Iowa is the only state that openly recognizes same sex partnerships as a form of common law marriage. This state does have spouse riders for this type of partnership. However, you may also be required to prove other aspects of your relationship like the ones mentioned above.
Each state has their own standards for determining a common law marriage that is based on age, consent and living arrangements. You will need to verify the rules that concern your state.