I copied a movieclip from one fla to another one. The copy itself seems to have worked but when I call mysprite.addChild(mymovieclip) I get Parameter child must be non-null.
Is there more that has to be done to get the movie clip created?
In the code you gave mymovieclip is a variable name, not a type. If mymovieclip is the linkage name try this instead:
var mc:mymovieclip = new mymovieclip();
Alternatively if you are trying to make a class for your symbol your class just needs to extend MovieClip and then you set that as the base class in the advanced properties window.