When I want to add a lookup column to a document library, created by and modified by columns are not listed. Can I add these columns to another list or document library as lookup column?
These fields are both of type
Person or Group. This field type is already an (inherited) lookup field (for the user list), and such it is not supported in a 2nd lookup field. See threads here and here. The second link contains a workaround via code, but I don't think it is a very good idea, as changes in the user list won't be refelcted in the 2nd linked list.