The following line must be commented out in /etc/samba/smb.conf:
username map = /etc/samba/smbusers
for the /etc/samba/smbusers file to be used to map Windows usernames with
the Unix/Samba usernames. Hence, when you add a user and specify the Windows
username, you can't log in from a Windows box using the specified Windows
username. 

The 'username map' option is commented out in the default smb.conf file, so
I see two options:

1) If you specify a Windows username when adding a Samba user, you need to either
enable the 'username map' option
2) Provide an option that enables/disables the 'username map' option and
if it is disabled, prompt the user to enable it if a Windows username is
provided when adding a Samba user. If this solution is chosen, the GUI should
make it clear that the 'Windows Username' is optional.