I think I got what you need.

You have a list of variables that is available at running time. It is stored in mavro variable varname.

for example varname contains a1 a2 a3 ... and you want to define a sum variable as
sumvar=sum(a1,a2,a3,....);

Here is a simple macro which will do it at running time.

For some macro string manipulating you can read my post

'shape' a macro variable into a desired look  

https://bbs.pinggu.org/thread-699918-1-1.html


***********************************************************

data t1;
retain an 1 bnj 2 avf 3 ah 4 a n b c 1;
output;
run;

%macro sum(dsin, dsout, varlist);

   %let sumvar=;

   %let n= %eval(%sysfunc(countc(%cmpres(&varlist),%str( )))+ (%length(&varlist)>0) );

   %do i = 1 %to &n;
     %let cur_var=%scan(&varlist,&i);
     %if &i=1 %then %let sumvar=&sumvar &cur_var;
     %else         %let sumvar=&sumvar , &cur_var;
   %end;

         data &dsout;
            set &dsin;
                sumvar=sum(&sumvar);
         run;
%mend;

option mprint;
%sum(t1, t1out, an bnj avf);
proc print;run;

%sum(t1, t1out, a n b c);
proc print;run;

%let v=an  avf  a  c;
%sum(t1, t1out, &v);
proc print;run;

非常感谢bobguy的答复，我的需要已经得到圆满答复！

学习了~~~