The Schmidt Easy Flow 9000 is a Parker ballpoint replacement that is supposed to be a capless rollerball. Now, as to whether or not it is a true rollerball, I'll let the experts argue over that. From what I have been able to find out it uses liquid ink like the rollerballs use. Due to a couple customer requests, I've been looking into this same issue the past week or two.
Monteverde also has a similar refill that they call a soft roll ballpoint refill. They come in a wide range of colors. I have yet to find out if these are liquid ink, but they state it uses the latest breakthrough in ink technology. I picked up a two refill pack at Office Depot and they do write very smoothly and feel like a rollerball.