You can add a SET rule to the Enterprise Service such that these inbound messages don't error in the first place.

Then define a condition, which should be that the tag does not already exist in the structure. Last step would be to set the value of the field to the static value.
However, if they do get to the Message Reprocessing application, you can adjust the payload of the message by pressing the pencil icon next to the message and directly editing the XML. From that dialog, press the Process button to re-process the message. Note that the status of the message needs to be HOLD for the edit and re-processing capability to work.
Hope this helps,
Alex
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Alex Walter
A3J Group LLC
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