I resolved this by doing a procedure from scratch based on GS1 SSCC spec doc.
Prob not the most elegant, but it works 
..
create procedure full_sscc
language sqlscript as
sscc nvarchar(17);
iSum int;
begin
select '39nnnnnn' || right('000000' || cast("U_SSCC" as integer), 9)
into sscc
from "@SSCC";
select
(substring (:sscc,1,1) * 3) +
(substring (:sscc,2,1) * 1) +
(substring (:sscc,3,1) * 3) +
(substring (:sscc,4,1) * 1) +
(substring (:sscc,5,1) * 3) +
(substring (:sscc,6,1) * 1) +
(substring (:sscc,7,1) * 3) +
(substring (:sscc,8,1) * 1) +
(substring (:sscc,9,1) * 3) +
(substring (:sscc,10,1) * 1) +
(substring (:sscc,11,1) * 3) +
(substring (:sscc,12,1) * 1) +
(substring (:sscc,13,1) * 3) +
(substring (:sscc,14,1) * 1) +
(substring (:sscc,15,1) * 3) +
(substring (:sscc,16,1) * 1) +
(substring (:sscc,17,1) * 3)
into iSum from DUMMY;
select '00' || sscc || cast(10 - mod (iSum, 10) as char(1)) from DUMMY;
end;