There is an option in the Fortinet 100D Software Policy to exclude Splashtop without manually adding all the IP's/Domains of the Relay Servers.
When setting a policy in Fortinet, click on "Policy & Objects", then click on "Internet Service Database".
Search for "Splashtop" and you should see an option to add Splashtop to the policy. The policy will contain all of Splashtop's IPs already making it unnecessary to add all the IP's manually.