You should consult a Calif. attorney, but, as a general matter . . .
1. probably not a good idea from the employer's point of view if, for example, the coverage is experience-rated, or self-insured, since it can end up costing the employer more than the "premium" that is reimbursed by the individual.
2. if they want to do it anyway, it would have to be permitted by the terms of the insurance policy (can't say that it covers "employees" only), and the terms of the employer's benefit plan.