From my research, it is my understanding that you can not do so. The employee can choose to leave the employer plan and move to a Medigap or Medicare Advantage plan, but you can not force them. It doesn't have so much do with ADEA as much as medicare rules. And then there is also the question on whether the employer can pay for either Part B premiums or the supplemental plan premiums.
"Medicare has special rules that apply to beneficiaries who have group health plan coverage through their own or their spouse's current employment. Group health plans of employers with 20 or more employees must offer these people the same health insurance benefits under the same conditions that younger workers and spouses receive. If your group health plan (participation is based on current employment) denies you coverage, or offers you different coverage, call your State Insurance Department" from http://www.medicare.gov/Choices/Employer.asp
"42 C.F.R. ยง 411.103 Prohibition against financial and other incentives:
(a) General rule. An employer or other entity (for example, an insurer) is prohibited from offering Medicare beneficiaries financial or other benefits as incentives not to enroll in, or to terminate enrollment in, a GHP that is, or would be, primary to Medicare. This prohibition precludes offering to Medicare beneficiaries an alternative to the employer primary plan (for example, coverage of prescription drugs) unless the beneficiary has primary coverage other than Medicare. An example would be primary coverage through his own or a spouse's employer..."