Can a DB plan be amended to provide for the purchase of past service credit for new employees coming into the plan by virtue of a merger of companies if the surving company did not otherwise give past service credit?

"purchase"?

Can Plan A be amended to merge in Plan B, and also recognize B employees' service prior to the merger date?
Yes. But suppose employees of A are already getting service only from plan inception date. Would the B employees get all prior service, even if prior to the plan inception date? (Employees of A might not be happy in that case.)

But you might be considering a different set of facts. If so, please post.

There is no plan A. Company A has a 401(k) plan. Company B has a 401(k) plan and a DB plan. Upon the merger, the employees of Company A will get service credit for service from the date of the merger (when they are employees of Company B).

401(k) plan A has a 3.5% contribution regardless of employee deferral and a dollar for dollar 6% match.

401(k) plan B has a 3% match only.

Long time employees of Company A hope to get more service credit under the DB plan. Hence the query about "purchasing" past service credit.

I think this is commonly done in some government plans.

Is this enough information?

If plan qualification requirements would prevent this kind of buy-in of service credit by participants, might it be accomplished with funding by the merged-in employer making contributions to the plan? Or, might it be accomplished through a separate plan? Non-qualified, perhaps?

Any ideas would be appreciated.