How to do more with movieclips?

Hello, I know Flash a lot, but I’m new to AS3 and Flex and am really lost with all that is possible to do.

I was wondering if there is a way
1- In flash to create a movie clip and then ADD frames in it to create an animation, OR to use an already existing movie clip and modify individual frames (for example, switch the bitmap in frame 3 from “bluePants” to “redPants” so the character now has red pants.

2- In Flex, to import bitmaps, make them assets in compile mode and then expert the SWF to import it in a flash movie and use the assets in movie clips.

Basically, what can you do more thant just importing assets? can you mix them up with movieclips and dynamically do something interesting??

thanks
Marco