This is the most Annoying bug on Android
For the weird times when your And says "facebook contact information is not editable on this device" the fastest way to add a phone number to a synched Facebook contact that doesn't have a listed phone number:
Contacts > select contact of interest > menu button > Edit Contact > menu button (again) > New Contact > enter the phone number > click Done
Phew! Wow, I know its kinda weird but that's the most efficient way. And its more intuitive that making a new dumbie contact and using the 'Join making a new dumbie contact and using the 'Join feature.