Can I add Samba users without having to add UNIX ones?

Here is a good explanation: http://ubuntuforums.org/showthread.php?t=825686

"The reason you need a unix account is so samba can restrict access to files according to permissions set on the filesystem. If the unix user doesn't have local permission to read a file, the samba user with the same name won't be able to read it either, regardless of the share's settings. I think you can disable the unix account, as long as the samba user is listed in /etc/passwd with a UID."

Check your [global] section. There is a security = user parameter. If you have this section set to = user you'll be asked for a password anyway. You have to change it to security = share, but note that this option was deprecated!