What you are describing is possible, though it wouldn't be the same sort of collective bargaining that is contemplated under the National Labor
Relations Act. It would be an ordinary contract and a "third party beneficiary claim." Example:
Employee X and employer Y contract to provide services under contract Z. The term of the contract is in perpetuity, and it provides that X will receive $10.00 per hour for all services, and that as further consideration for the agreement Y agrees that any other employees hired during X's employment shall be paid at the same rate.
Under a third-party beneficiary theory, other employees who are hired can sue if they are not paid at least $10 per hour, once they have notice of the agreement between X and Y. This is a well established legal doctrine -- but it's not collective bargaining, per se, because: (1) under federal law, a union cannot bargain on behalf of employees until the union is formally elected using the required process to validate the union's authority; and (2) the other employees who are the beneficiary of the original agreement didn't collectively bargain for it in any way. They weren't even aware that the contract existed.
The full answer to your question will be found within the "four corners" of the contract that you are seeking to enforce -- because the contract cannot be expanded into a union contract, merely because it has the look and feel of a union. Federal law preempts any attempt to unionize under rules other than those provided by the National Labor Relations Board. Which is why I'm suggesting that this contract would be either a third party beneficiary agreement, or it would be void as against federal law.
Hope this helps.