Keep Values in Message Mapping

Hi,

maybe someone can help me. I want to know if there is the possibility to keep values in the target message during a message mapping.

So for example if the source message has an empty field but the target message has a default value for that field, is it possible to have the default value in the target message?

The whole problem: I have two messages of the same type and structure (first not fully filled but with new data, second fully filled but with old data) and want to merge them to have all new data and the empty fields of the first message filled with the old data. But mapping one message on the other just erases all data from the target message. So I'm looking for a way to get around a multi mapping where I have to insert an "If" in every field-mapping (more than 1000 fields).

Is there another possibility?

Best regards

Robert