Hi,
The Additional data (targetData) field is XML, so can't be updated via Update activity and is harder to use in general.
Simple fix is modify the nms:seedMember schema to turn it into a real boy, i.e. <element name="targetData" xml="false"/> and continue as normal. May/may not blow up MTA.
More complicated fix is use js to set the mData col to wrapped xml from the file.
Thanks,
-Jon