To project a derived column, add a join to your organisation table and derive the new column based on the various values:
SELECT T.[taskId], T.[sectionId], T.[DateAdded],
CASE
WHEN SectionId IS NULL THEN 1
WHEN o.[type] = 6 THEN 2
ELSE 3
END AS [type]
FROM [TASK] AS T
-- Applicable join condition goes here ...
LEFT OUTER JOIN Organisation o ON t.TaskID = o.TaskID
WHERE (T.[deleted] = 0)
ORDER BY T.[DateAdded]