Employer's bargained employees are covered under a multiemployer plan 401(k) ("Plan A") pursuant to an area collective bargaining agreement. Plan A is terminating and Employer has no say in process b/c it is not a bargaining party. Bargaining parties decide to sign on to new area multiemployer 401(k) plan ("Plan B"), forcing Employer to allow collective bargaining employees to participate. Is Plan B a successor plan to Plan A as to the Employer? Is there an argument Employer has not established or maintained the plan under the (k) regs?

What is the purpose of your question? For what purpose are you trying to determine successor plan status? Does it relate to potential distribution to participants pursuant to Plan A's termination?

Yes, from A to B via rollover.

Look at Treas. Reg. Section 1.401(k)-1(d)(4)(i) to see if the 2% rule applies. If not, then you do not have a distributable event, because Plan A would be an alternative dc plan maintained by the employer (& other employers, as well) and therefore it would not have terminated for purposes of the 401(k) distribution rules. I don't see an exception for a collectively-bargained plan.

You'd just have to transfer all assets for the employer's union employees from Plan A into Plan B and not permit a lump sum distribution (ior any other distribution) for those who have not yet terminated employment.