The Transfer Tool does not transfer forward domains from Plesk.
In order to transfer forward domains from Plesk to cPanel you'll need to convert them into full domains first. You can do this with the following command. NOTE - Please repace the example portions of the command with the details specific to your domain:
/usr/local/psa/bin/domain --update example.com -hst_type phys -login "example" -hosting true -ip 127.0.0.1 -passwd "SET_YOUR_OWN_STRONG_PASSWORD"