Yes, but you'd have to enter each contact one at a time.
On your Today screen look at the icons at the far right at the bottom of your screen. Tap the SIM card icon to open the app. At the bottom of the screen tap "New" and manually enter the contact. Hit "Save" and it will be saved to your SIM card.
To date, there is no way to save your entire phone book to your SIM all at once. It's a limitation in the firmware and Windows Mobile Pocket PC Phone Edition. Hopefully it will be solved with the new Windows Mobile 5.0 release... but of course the 63xx series won't be seeing it!!
