yes you can set a loopback address on a router in Packet Tracer ver 3.2
in topology view, while not being in Simple Mode, you need a place a router, go into it, engage the console.
after that you need to turn on priviledged command by typing 'enable' and pressing enter.
after that you need to enter into configuration mode by typing 'configure' and pressing enter.
this now will let you configure any interface, router list, and ofcourse a loopback address as well. its just like programming up a router using a console.