What is keeping you from adding more? Have you tried hitting return after the last address field is filled? More fields keep magically appearing. A scroll bar will appear to indicate that you have more fields than are being displayed. If you just have to see more fields, move your mouse to the bottom of the subject field and watch for it to change to a double headed arrow. When it does left click, hold and drag down to reveal as many address fields and your have screen to display.

thank you, thank you, thank you; I didn't know until you told me.