Yes basically its possible , because Forti-Authenticator is radius server .
What type of the authentication you are using (customer requirement , 2 factor , Social or user name and password )
as per my knowledge flow like this ,
1. on Wifi controller add Forti-authenticator as the radius server
2.in fortiauthenticator create wifi controller as the radius client
3.in aruba capative portal config redirect authentication to external server
please check these links for reference
https://community.arubanetworks.com/t5/Wireless-Access/External-Captive-Portal-Authentication/td-p/236949
https://cookbook.fortinet.com/fortiauthenticator-guest-portal-fortiwlc/