you could leverage function catx as below. catt function is very useful and convenient.

    data a;
    input id @@;
    cards;
    1 2 3
    ;
    run;
    data b;
    set a;
        length new_id $10;
        new_id=put(id,1.);
        do i=1 to 1+_n_;
          new_id=catx(',',new_id,id);
        end;
    run;

Even simpler as,

data b;
    set a;
        length new_id $10;
        do i=0 to 1+_n_;
          new_id=catx(',',new_id,id);
        end;
    run;