Instead of using "NULL" inside the openquery query string, use it outside. In other words something like

SELECT  HAAPRO-ACTIVE, ......, CONVERT(VARCHAR(100),NULL) AS HPMASN-HRS-PER-DAY-DEC
 FROM OPENQUERY([SKYWARD1],.........<do not list the HPMASN-HRS-PER-DAY-DEC inside the query>.......
UNION
SELECT .....