You could use the USER1 thru USER4 fields towards the bottom of the contact screen but when you try to call the contact the number in the USER field will not appear as an available number to automatically dial. You can also change the name of these fileds by scrolling to the field, pressing the trackwheel, and selecting Change Field Name. If you're just trying to keep track of numbers this may work for you but if you actually want to automatically dial the number this option won't work.