Sorry. The GEN 4 (2004 LS430) system does not allow for you to manually input a contact. Only GEN 5 systems allow for users to add new contacts. Your best bet is to check LetsTalk.com for Lexus and find a handset that is more compatible with your vehicle in comparison to your current handset.
Finally an answer - this is brutal that you can't enter #'s manually one at a time. I have a '06 LX470.