Yes, you can use the 'DO' object in the general folder. In your example you are better of using a 'set'.

Use your date object in the set and type this in the expression editor:

{
 String s_date = "20091231";

 int year = int.valueOf((s_date.substring(2,4)));
 int month = int.valueOf((s_date.substring(4,6 )));
 int day = int.valueOf((s_date.substring(6)));
 
 return new Date(year,month,day);
}

If you want to put it in a coulple of lines of code you could do this:

{

 String s_date = "20091231";

 return new Date(int.valueOf((s_date.substring(2,4))),int.valueOf((s_date.substring(4,6 ))),int.valueOf((s_date.substring(6))));

}

Note: You have to have all you code surrounded by { }

Cheers

Jason