For schematic purpose, you can create anything you want there is a built in symbol editor in Multisim. The following tutorial shows how you can create your own component with simulation model, if don't have a model, you check the "Layout only" option on the first step and the Component Wizard will skip the import model steps. http://www.ni.com/tutorial/3173/en/
Unfortunately, Atmega is not planned to be supported yet.
