data test;
input x $ y;
su=0;
cards;
.
.
.
;
run;
proc sql noprint;
select sum(y) into:s from test;
create table test1 as select * from test;
update test1 set su=&s;
quit;

多谢！算了一下，结果正确！不过程序还没懂，要好好学一下了！

 thanks lot， rain958!

我也是现学现卖.
这个更好些.
data test;
input x $ y;
cards;
.
.
.
;
run;
proc sql noprint;
select sum(y) into:s from test;
alter table test add s num;
update test set s=&s;
quit;

大家想的太复杂了，再data步里加一个语句就行了！~
data test;
input y;
s+y;
cards;
1
2
3
;
run;

楼上正解,用变量累加