Look up Knurl Rollerball. I did a simple conversion of a Knurl GT to Rollerball. I used a Pentel refill that has an ink works in Click and twist pens. Not sure how a higher end Rollerball refill will fair with no cap.
Talked to end user yesterday, a Nurse who has went through the first refill on one of the two pens and no issues with ink drying out or with the twist or spring. Still quite happy.
A Parker style ballpoint will accept a parker style gel refill. i have found the mountverde and parker quink gel refills to write just as smoothly (even more wet) than a rollerball. the on;y downside is the the cost; the gel refills are about $4.
The ones she uses are the Pentel EnerGel .7mm refill. I think she gets a box of 12 for about $12. I expect nursing is one of those professions that they still use a lot of ink. It is a gel with a proprietary ink and "a Rollerball feel" , making me think it is not what the industry considers a true Rollerball. It does have a Rollerball shape to it.