There are kits on ebay such as the one below. You might have to make a few changes to your machine layout. One simple thing you could do is set up endstops so that the bit/endmill can not possibly touch any of the 4th axis stuff. I am not quite sure how you would go about setting up Gcode for it though. You might need to upgrade to something like mastercam and possibly different firmware on the ramps board or a more industrial stepper driver system. But I don’t see why the ramps couldn’t do it as long as you feed it the Gcode in a way that it can make sense of it.
Our Haas VF1 at work as a 4th axis but we usually don’t even keep it in the mill because it takes up space that is better used by a vise. We really only set it up (which takes forever) for round stock like you said. I actually have to make a part here at school that needs a 4th axis and I haven’t found a mill yet that has one. We might end up rigging something up with pillow blocks and just spinning the stock by hand.