An easy way to do it is to view your results as a table itself. Then group by on that and SUM. For example, wrap your existing sql as a derived table:
SELECT SUM(x.CurrentOrders), x.CurrentYear
FROM (
(SELECT SUM(Amount) AS CurrentOrders, YEAR([Order Date]) AS CurrentYEAR
FROM [Sales Header]
WHERE ([Document Type] = 1) AND (Code4 = 'SLAM')AND ([Currency Code]= 'DKK')
Group BY YEAR([Order Date])
)
UNION ALL
(
SELECT SUM([Outstanding Amount ($)]) AS CurrentOrders, YEAR([Order Date]) AS CurrentYEAR
FROM [Sales Header]
WHERE ([Document Type] = 1) AND (Code4 = 'SLAM')AND ([Currency Code]<> 'DKK')
Group BY YEAR([Order Date])
)
) x
GROUP BY x.CurrentYear