You can have a look at this tutorial for more info:
http://www.republicofcode.com/tutorials/flash/as3displaylist/
but adding an object on top of another one , is as simple as this
addChild(child1);
addChild(child2);
basically the first object added is at the bottom of your stack. now if the object you want placed under was added after the first one. you still can do this:
addChild(child2);
addChildAt(child1, 0 );
there are, of course ,more options to manipulate the children positions but you should find what you need in the tutorial above.
Oops , sorry the word Canvas , got me confused. I thought you were trying to do this within Flex. Do you mean to say you want to do that in HTML. If you do, please retag your question, because it wouldn't really be a flash issue .
Hope it helps ;)