Help & troubleshooting for channels on your Roku device, including adding/removing channels, logging in to, authenticating, or activating a channel, channel-specific playback issues, assistance contacting channel publishers to report issues, and adjusting channel-specific settings.
You should still be able to use the Xfinity app. It's currently free, but Comcast has noted that when it comes out of "beta" testing, they may charge as if it was another cable box. I assume this is an extra box, and not the single one that is usually included with most packages.
I would try the Xfinity app on the Roku first to see if it works the way you want it to.