Hi,
As far as I know...there is only one solution...its because how cPanel authenticates FTP.
You can accomplish this using the following steps,
1. Assign a dedicated IP to the domain
2. Set the FTP host as the particular domain (or *.domain pointing to that IP ) or the dedicated IP in your FTP client.
Now you can access the additional user just as user rather than user@domain.
Thank you,
Nibin.