この SQL クエリと同じ結果を得るための NHibernate フィルターは次のとおりです。
SELECT A.Id as ItemId, A.column1 as ItemTitle, A.ListId as ItemListId,
B.Id as LookupItemId, B.Value as LookupItemValue, B.ListId as LookupItemListId
FROM MyTable A LEFT JOIN MyTable B ON B.Id = A.LookupId
WHERE A.ListId = 'EmployeeListGuid'
AND (B.ListId='EmployeerListGuid' OR B.Id IS NULL)