I stand to be corrected ... I don't believe the Knurl GT is available as a rollerball, only as a twist ballpoint.
I have never tried to fit a rollerball refill to a ballpoint pen but it might be possible.
I have done it the other way around, but a slight modification to the internals of the pen is necessary, and you don't have retractability.
If you want to try fitting a rollerball refill to a Knurl GT, one of the problems you will face is that the ink will probably dry out fairly quickly ... the rollerball refill ink is designed to be used in a capped pen to prevent the drying of the ink (or slow down the drying). Another problem will be the possible leaking of the ink onto clothing, etc., even when the refill is retracted, as the ink is more fluid.
The rollerball refill is very nearly the same diameter as the ballpoint refill, so no problem in that regard. . But the rollerball refill is noticeably longer, so you would have to trim off part of the plugged end of the RB refill ... remove the little plug and shorten that end of the refill, then replace the plug.
I don't know if the shape of the RB refill top end will properly allow extension and retraction by the ballpoint click mechanism.
Another problem is that the writing end of the RB refill is not designed to accommodate a spring as required for ballpoint operation, but you might be able to add a short, narrow sleeve to the RB refill that would make this possible.
Please let us know any ideas you come up with ... I would be very interested.