You said "if they want to add files/directories to that drive they have no choice but using FTP". This sounds like they should be able to read via shell access. So you export it to another box for read access via shell accounts. If you mean ftp only, no shell accounts ever allowed for any purpose, that is easy. Make their login shell /bin/true. You probably then need to add /bin/true to /etc/shells so that ftp will work depending on what ftpd software you use. Accounts like this are ftp only. Do not add /bin/false to /etc/shells. Then you can change the shell to /bin/false to disable ftp access.

If you have some other requirement, express it more clearly.