Yes, this is possible. You can create the subdomain from within your subscription. Afterwards, create an additional FTP access by clicking on the "FTP" icon in "Websites & Domains". As a "home" directory of that FTP access account, enter the sub domain's directory (e.g. /mysubdomain.domain.tld). This will jail the FTP user to that very subdirectory. An FTP user account like that will not have access to other settings of the domain, e.g. mail settings.