Hi Tom,
it sounds a bit like the new field is not set to be "changeable" in the BuilHeader-Model. Maybe the following steps solve your problem:
1. TX GENIL_MODEL_BROWSER
2. Component Set ALL
3. Open the Root Objects > Root Object BuilHeader
4. Open Attribute Structure
5. Double click your new attribute ZZAFLD00000Z
6. Attrib. Property might be N (not defined) but should be C (changeable)
Hope it helps.
Best regards.
Ben