Here's a macro I use for such things:
/*******************************************************************************
*  Name:   lisuffix  (List Item Suffix)                                        *
*  Desc:   Returns a list with the specified suffix appended to each item.     *
*  Type:   Macro Function - List                                               *
*  Usage:  %lisuffix ( list , suffix )                                         *
*  Where:  list is a space delimited list                                      *
*          suffix is any string of characters                                  *
*  Examples:                                                                   *
*    1)  %let fylist=1996 1997 1998;                                           *
*        %let newlist=%lisuffix(&fylist,%str(,));                              *
*        %put ===> newlist=&newlist;                                           *
*        ===> newlist=1996,1997,1998,                                          *
*    2)  %let datelist=01JAN1999 01FEB1999 01MAR1999;                          *
*        %let newlist=%liprefix(&datelist,%bquote('));                         *
*        %let newlist=%lisuffix(%bquote(&newlist),%bquote('d));                *
*        %put ===> newlist=&newlist;                                           *
*        ===> '01JAN1999'd '01FEB1999'd '01MAR1999'd                           *
*  History:                                                                    *
*   3/15/00   Walt Smith - development                                         *
*******************************************************************************/
%macro lisuffix (_list,_suff) /
   des = 'Fn: Suffix each item in list with string'
   ;
   %local return i signal wrd delim;
   %let delim=%str( );
   %let i=0;
   %let signal=CONTINUE;
   %do %until(&signal=DONE);
      %let i=%eval(&i +1);
      %let wrd=%qscan(&_list,&i,%str(&delim));
      %if %length(&wrd)=0 %then
         %let signal=DONE;
      %else
         %let return = &return.%str(&wrd)%str(&_suff)%str(&delim);
   %end;
%unquote(&return)
%mend;
You need to be aware of macro quoting - if what you want to append to each item in your original list contains anything that is meaningful in calling a macro (like = signs, commas, etc) then just quote the whole thing. In your case, as I read your question you want to add ', (n, mean, var)' to each of x1 x2 x3:
%let ind_variable = x1 x2 x3;
%let newstr = %lisuffix( &ind_variable,  %bquote(, (n, mean, var)) );
%put newstr = "&newstr";
result:
newstr = "x1, (n, mean, var) x2, (n, mean, var) x3, (n, mean, var)"
ouch! I just previewed this post and they filter out all whitespace so the macro looks pretty ugly - sorry ...