Without further due, here is a hack replacing the Zebra F-301 F refill with the one from Uni-ball Jetstream 0.5mm. Get the smaller Jetstream refill, usually the one to refill the Uni-ball multipen.
The next step is to open the black cap from the F refill. This is easily done using your nail. Then you’ll have to take out the refill from inside the barrel. This also can be done by hand, just push down-up.
Next, you’ll have to do a hack to put the Jetstream refill into the F refill barrel. You can use your creativity. What I did was cutting a bit of the black cap from the F refill, and some part of the Jetstream plastic (same length), and then wrapped the middle with some tape, to keep its position in the barrel.
Now put them altogether into the F-301 body, and Voila! This is better and cheaper than my Parker.