As far as I understood Centova V3 uses a modified pure-FTPd daemon.
Correct.
It seems like you are using different syntaxes for cc-ftpd config file.
Pure-FTPd does not support configuration files at all, it exclusively uses commandline parameters. There's a perl script available to translate a text file into commandline parameters which is probably what you're thinking of, but we don't use it as it would require adding perl as a dependency for Centova Cast (which is not needed anywhere else).
Our "configuration file" is just a BASH script where you set some values to be passed to Pure-FTPd.
Is it possible to add similar line to cc-ftpd.conf like "PassivePortRange 30000 50000"?
It's not practical to reimplement every commandline option in cc-ftpd.conf, but I will add a FTP_OPTIONS setting to allow you to provide custom parameters to Pure-FTPd more easily than editing /etc/init.d/centovacast directly.
After the next build, you can just add FTP_OPTIONS="-p 30000:50000" to cc-ftpd.conf and restart.