Do I need her biological fathers consent to add my last name to hers? He father hasn't seen her in almost a year and won't agree to it. I don't want her to lose his last name just add mine because that how my youngest daughter name is. She keeps asking me why her sister is so lucky. He hasn't paid child support in over 2 years and I'm not pursing it anymore. I don't have an address for him and yes he is on the birth certificate. I live in cook county.