"2013DEC02" is not a string in date9 format.
Are you dealing with a SAS date value (=numeric variable containing a number representing the number of days since 1 january 1960) or are you dealing with a string?
Should the result be a string or is this simply about applying a date format to a SAS date value?
Below code gives you examples for both cases.
data demo;
DT_String_In='2013DEC02';
SAS_DT_Value_no_date_format=input(DT_String_In,anydtdte.);
format SAS_DT_Value_with_date_format yymmddn.;
SAS_DT_Value_with_date_format=input(DT_String_In,anydtdte.);
DT_String_Out=put(SAS_DT_Value_no_date_format,yymmddn.);
output;
DT_String_Out=put(SAS_DT_Value_with_date_format,yymmddn.);
output;
run;