How to create a values set with query - to get Difference FTE as value

I want to create DFF field with value set - table ?


SELECT Positions.name "Position Name",
Positions.POSITION_CODE "Position_Code"
,Positions.FTE "Position Current FTE"
,Positions.INCUMBENT_FTE "Current Incumbent FTE"
,(Positions.FTE - Positions.INCUMBENT_FTE) "Difference FTE"
FROM
(SELECT HAPFT.NAME,
HAPF.FTE,
HAPF.POSITION_CODE,
(select SUM(PAWMF.VALUE)
from PER_ALL_ASSIGNMENTS_M PAAM,
PER_ASSIGN_WORK_MEASURES_F PAWMF
where 1=1
AND PAAM.POSITION_ID = HAPF.POSITION_ID
AND SYSDATE BETWEEN PAAM.EFFECTIVE_START_DATE AND PAAM.EFFECTIVE_END_DATE
AND PAAM.ASSIGNMENT_TYPE = 'E'
AND PAAM.ASSIGNMENT_STATUS_TYPE = 'ACTIVE'
AND PAAM.ASSIGNMENT_ID = PAWMF.ASSIGNMENT_ID