Hi Engin,
this won´t work as you can only change the content for existing fields in the document, but not adding additional fields.
You need to make sure that the fields have been added by earlier steps in the process instance.
When this is a design purpose you can build a new version of the BPM and deploy it to PE/PRT.
When this is done you can update the instances to use the new version instead of the old version.
But I am not sure if this will fill the new fields automatically then.
See BPM Developers Guide for further informations.
Regards,
Holger
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