Hello,
You need to customize the Document library web part, so in the \CMSModules\DocumentLibrary\Controls\DocumentLibrary.xml file please add your custom column like:
<column source="NodeGUID" externalsourcename="someFlag" caption="MyField" wrap="false" />
You need to also add the someFlag case into the gridDocuments_OnExternalDataBound method of the \CMSModules\DocumentLibrary\Controls\DocumentLibrary.ascx.cs file.
Best regards,
Jan Hermann