You can not "buy-into" the Certified Pre-Owned program. BMW dealers use a loophole in the rules to certify vehicles that they have originally sold/leased. You can not certify a vehicle that was bought through a third party under any circumstances, and I believe you can not go to any other dealer than the one that originally sold the vehicle.
Of course the way to get around this is to have the dealer buy the car, certify it and sell it to you, but you will spend thousands extra.
The certified program is the prefered extended warranty program because it goes through the BMW database. There is no paperwork to carry around, no headaches in finding a place to accept the plan, no problems with only paying a certain amount for a repair, etc. It is also much easier for the dealer in that it is done just like the new car warranty. There are no special procedures for claims or payment, and there is no need to call in for authorization and payment. There is one $50 deductible per visit (regardless of number of repairs) and there is no tax, miscellaneous fees, etc. that you will be hit with.