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

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

 

Add this line:

 

Creat a crypted password

 

Edit the user file like this:

 

Example

 

And finaly, reload proftpd