Why do you think you can't do this? Just enter an address, press return. A new line is created. Add the next address. Repeat. If you use the Contacts Sidebar, you can multi-select several addresses and use the "add to..." buttons to add them. When you have added some addresses, you can scroll up and down the list by use of the mouse or via the arrow keys. You can also grab and drag the top edge of the message text pane and move it down to make room to display more of your addressees.