There should be a jack on the back of your box that says IR Input or External IR. You simply then need to likely plug in something like a Xantech IR receiver into that jack. Confirm the pinout with Comcast please.
Now comes the fun part. You somehow have to find out what IR codes they are using. That is where an RF to IR converter comes in handy.
Before purchasing the bridge you mention, I would try out some of the IR codes here for the motorola or other brand cable boxes to see if they work with your equipment.
I am hoping others weigh in here as I have FIOS and not Comcast and NO RF remotes.