consulta saldos vencidos

buenas tardes foro

estoy realizando una consulta para que me mande una alerta de las facturas vencidas a la fecha.

la consulta es la siguienete

SELECT T0.CardCode AS 'supplier code',

T0.CardName AS 'supplier Name',

CASE

When T0.DocType = 'I' AND T0.DocCur = 'USD'

then SUM(T0.DocTotal) - SUM(T0.PaidSys)

when T0.DocType = 'S' AND T0.DocCur = 'USD'

then SUM(T0.DocTotalFC) - SUM(T0.PaidSys)

When T0.DocType = 'I' AND T0.DocCur = 'MXN'

then SUM(T0.DocTotal) - SUM(T0.PaidSys)

when T0.DocType = 'S' AND T0.DocCur = 'MXN'

then SUM(T0.DocTotalFC) - SUM(T0.PaidSys)

else 'NULL'

end AS 'Purchase Type',T0.DocCur

FROM OPCH T0

WHERE DATEDIFF (DD, T0.DocDueDate, GETDATE( )) BETWEEN -7 AND 365

AND T0.DocStatus = 'O' AND T0.SERIES = 10 AND T0.CANCELED = 'N'

GROUP BY T0.CardCode,T0.CardName,T0.DocType,T0.DocCur

ORDER BY T0.CardCode,T0.CardName,T0.DocType

FOR BROWSE

mi cosnulta es como puedo agregar mas casos si la estructura de mi  When ya la estoy ocupando.

EJEMPLO :

when T0.DocType = 'S' AND T0.DocCur = 'MXN'

then SUM(T0.DocTotalFC) - SUM(T0.PaidSys)

y requiero Utilizar otra asi:

when T0.DocType = 'S' AND T0.DocCur = 'MXN'

then SUM(DocTotal) - SUM(PaidSys)