You can also leave off the constraint and add a FK at the query level within the server using the metadata xml:

    <ns:column columnName="ResponsiblePersonId">
        <ns:columnTitle> Responsible Person</ns:columnTitle>
        <ns:nullable>true</ns:nullable>
        <ns:datatype>userid</ns:datatype>
        <ns:scale>30</ns:scale>
        <ns:fk>
          <ns:fkColumnName>UserId</ns:fkColumnName>
          <ns:fkTable>Users</ns:fkTable>
          <ns:fkDbSchema>core</ns:fkDbSchema>
        </ns:fk>
      </ns:column>