We already have this option when creating a Search index. Take a look at the attached image. To use the search, you need to specify where your forums are published. Also, on the Search page you should have a Search box and a Search results control, where the Search box is configured the use the Search index, which indexes forums.
All the best,
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