There is no other way to add permission to forum post programmatically. The internal class ForumsPermissions is the access point to those permissions. I have logged a feature request
to allow programmatic creation of permissions for forums.
the Telerik team
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items