You can wrap your results in a CTE and then use it to sum the totals like this:
with results as
(
select code,name,SUM(SINETT) AS TOTAL from #tableSI GROUP BY CODE,NAME
UNION
select code,name,SUM(SINETT) AS TOTAL from #tableSR GROUP BY CODE,NAME
UNION
select code,name,SUM(SINETT) AS TOTAL from #tableRC GROUP BY CODE,NAME
UNION
select code,name,SUM(SINETT) AS TOTAL from #tableDN GROUP BY CODE,NAME
UNION
select code,name,SUM(SINETT) AS TOTAL from #tableCN GROUP BY CODE,NAME
)
select * from results
union all
select '' as code, 'grand total' as name, sum(total) as total from results
This will give you this result:
code name total
----- --------------------- ---------------------------------------
DT001 DAKNA TRADING SDN BHD -96545.61
DT001 DAKNA TRADING SDN BHD -1590.00
DT001 DAKNA TRADING SDN BHD -689.00
DT001 DAKNA TRADING SDN BHD 1060.00
DT001 DAKNA TRADING SDN BHD 94263.21
grand total -3501.40