Plesk ProFTPd sever has compiled-in mod_tls.c module for SSL support ans SSL support can be configured in /etc/proftpd.conf for example in the following way:

<IfModule mod_tls.c>
TLSEngine on
TLSLog /var/log/tls.log
TLSProtocol SSLv23
# Are clients required to use FTP over TLS?
TLSRequired off
# Server’s certificate
TLSRSACertificateFile /usr/local/psa/admin/conf/httpsd.pem
TLSRSACertificateKeyFile /usr/local/psa/admin/conf/httpsd.pem
# Authenticate clients that want to use FTP over TLS?
TLSVerifyClient off
# Allow SSL/TLS renegotiations when [...]