Providing your channel has an average of 40 or more users, you can.

You must first talk to a staff member in #help and request the forum be added. Providing you meet the above requirement, a forum will be added for your channel. We will also add a user group of moderators for your forum. This can include up to 5 registered forum users. You may request a single change to this moderator group every 30 days. A single change is counted as adding a user, deleting a user or swapping one user for another.

If you have any questions, come see us in #help.