I can add private tag in existing dicom header using dcm4che2 : -
private int privateTagForOldData = 0x78610010;
private int privateTagCreator = 0x78611010;
dicomObject.putString(dicomObject.resolveTag(privateTagForOldData, "Test", true), VR.LT,
"private tag description");
LOGGER.info("Private tag added");
How to do using dcm4che3, as method resolveTag doesn't exist
attributes.setString(ds.resolveTag(privateTagForOldData, "Test", true), VR.LT,
"private tag description")