There isn't a way to do this. If you have an MXML file you
can use the <mx:Script source="whatever.as" /> tag to include
code from another file, but for a pure class definition
ActionScript file, it is not possible.
I also think it is unwise. First, you would lose the Flex
Builder code parsing, code hinting and outlining which can make
navigating your class files easier. Second, it would turn into a
maintenance nightmare. Imagine you had 2 files for every class and
had dozens of classes. If you had to ask someone to look at your
code it would be very difficult and frustrating to open the class
file and then to open additional files to look at the code; those
additional files would not have import statements to help identify
the origins of the classes.