ProFTPD is the FTP software that current versions of Plesk ship with, and thankfully found what I was looking for.

ProFTPD allows you to create multiple virtual users by using an AuthUserFile directive. The AuthUserFile directive sets the path of a textual file containing a list of users and passwords for authentication.

 

Create a user file

$ touch /etc/proftpd.authuserfile

Edit the ProFTPD configuration file. (NOT etc/proftpd.include as Plesk overwrites that file regularly.):

$ vi /etc/proftpd.conf

 

Add this line:

AuthUserFile /etc/proftpd.authuserfile

 

Creat a crypted password

$ openssl passwd -crypt passwordhere

 

Edit the user file like this:

username:password_encrypt:uid:gid:gecos:home:shell

 

Example

toto:aASDFajksASa:10022:2221::/var/www/vhosts/toto.com/httpdocs/data/web:/bin/false

 

And finaly, reload proftpd