Yes. If they qualify as a legal dependent under the Plan Rules.
A “Dependent” is your lawful spouse.  However, in accordance with state law, effective July 1, 2005, if you are enrolled in one of the Prepaid Medical plans, your dependent may include your domestic partner of the same sex or opposite sex (effective January 1, 2020), age 18 or older.
To qualify for domestic partner coverage, you must have registered as domestic partners with the Secretary of State and submit required proof of the relationship in accordance with the law. Your domestic partner’s children are not eligible for coverage.
Effective September 1, 2014 your dependents also include your dependent children to their 26th birthday.  Legally adopted children and step-children are also considered eligible dependents.
A dependent over age 25 years of age may continue to be eligible as a dependent if he/she is incapable of self-support because of physical or mental incapacity that commenced prior to reaching the age of 26, provided a physician’s certification of disability is submitted within six months following their 25th birthday or the effective date of eligibility.
A dependent who is eligible for group benefits as an employee or as the dependent of any other employee other than one of his parents is still eligible until his or her 26th birthday.  Proof as an eligible dependent is required.