Thanks again perestrelka.
I filed a suppport ticket, and it looks like the additional FTP accounts is via the Web Users functionality:
1) Log into Plesk
2) Click on "My Domains"
3) Click on your domain name (in my case ponju.com)
4) Click on "Web Users" located under "Hosting"
5) Click on "Add new Web User"
6) Supply the Name and password
7) Click "OK"
The Web User that can be created in Plesk will be limited to its directory in /var/www/vhosts/ponju.com/web_users
So for what I want, which is to create a FTP user that has a read/write privileges to a path in /httpdocs/custom/ directory, it seems like I'd have to use the SSH method.