没有想到更好的方法，楼主可以试试下面的代码：

DATA TEST;
INPUT ID PROD$;
CARDS;
11 A
11 B
11 C
21 B
21 C
31 D
;

proc sql;
   create table tmp1 as
    select distinct * from
         (select distinct id from test)
         ,(select distinct prod from test);
quit;

data tmp2;
   merge tmp1 test(in=a);
   by id prod;
   value=a;
run;

proc transpose data=tmp2 out=wanted(drop=_name_);
   by id;
   id prod;
   var value;
run;

proc transpose data=test out=tmp1(drop=_name_);
   by id;
   id prod;
   var id;
run;

data wanted;
    set tmp1;
        array tmp(*) a--d;
        do i=1 to dim(tmp);
           tmp(i)=^missing(tmp(i));
        end;
        drop i;
run;

这种方法很好，多谢多谢