I found this information on the O'reilly book website:
184.108.40.206 Adding entries to smbpasswd
There are a few ways you can add a new entry to the smbpasswd file:
You can use the smbpasswd program with the -a option to automatically add any user that currently has a standard Unix system account on the server. This program resides in the /usr/local/samba/bin directory.
You can use the addtosmbpass executable inside the /usr/local/samba/bin directory. This is actually a simple awk script that parses a system password file and extracts the username and UID of each entry you wish to add to the SMB password file. It then adds default fields for the remainder of the user's entry, which can be updated using the smbpasswd program later. In order to use this program, you will probably need to edit the first line of the file to correctly point to awk on your system.
In the event that the neither of those options work for you, you can create a default entry by hand in the smbpasswd file. The entry should be entirely on one line. Each field should be colon-separated and should look similar to the following:
This consists of the username and the UID as specified in the system password file, followed by two sets of exactly 32 X characters, followed by the account flags and last change time as it appears above. After you've added this entry, you must use the smbpasswd program to change the password for the user.
You may want to note that the directories for redhat are not the same as the ones in the paragraph above. On my box, the bin directory for Samba is /usr/bin, not /usr/local/samba/bin. It depends on whether you use a stock samba setup from your distro or download samba from the internet as to where it sets up.