Add order to existing shipments automatically

Hello,

Looking for any ideas around a way to automate orders coming in through integration or while in a bulk plan to first look for feasible existing shipments and then if found add them to those shipments.  We cannot lose the shipment id from the first shipment.

Scenario.  Two order drop, first bulk plan all is good.  Second order drop later in day, those orders need to see if there is a ship from first drop that they can go on.