There will be times where 6 Object Response Channels and 2 Trace Response Channels simply are not granular enough for what you want to create. This is what the Collision Editor in your Project Settings is for. You can access them from the Edit Menu -> Project Settings -> Collision:
From here you can add new Object Response Channels and Trace Response Channels. Click the New Object Channel... or New Trace Channel... button, provide a name, select a Default Response, and click Accept.
You can have up to 18 Custom Object Response Channels or Custom Trace Response Channels.