The simplest way to set up a nested hierarchy of symbols in flash is to add symbols inside one another.
A quick example.
Draw a circle on the stage, select it.
With the circle selected make it into a movie clip or graphic (F8 or Modify > Convert to symbol)
Accept the defaults in the convert to symbol dialogue
Double click on your circle symbol on the stage to 'enter' it.
Add a new layer to keep things neat.
On that new layer draw another circle, and make that into a symbol too.
You now have one circle symbol nested inside another.