Could you be more specific of what is the option name to customize FTP Welcome message in WHM ?

I did not find anything...

More specific than what? :confused:

Cpanel has an anonymous FTP message control in the FTP manager of account settings.

Want to configure its welcome message?

Simple, just edit the following file:

/etc/proftpd.conf

Add "ServerIdent" this, or modify it:

EX:
ServerIdent on "ProFTPD FTP Server ready."

Etc, it is good when you want to have resellers, that way you can hide the hostname of your company in the FTP session.