Rapidograph pens are meant to be used with disposable ink cartridges, but that does not mean you can't refill them yourself.
A box of 3 ink cartridges can cost upwards from US$6. Each cartridge has around 1ml of ink. A Rotring 23ml ink bottle cost around US$9. It's just more economical to refill it yourself.
These are the tools that you need: Ink, pen, needle and syringe. The needle and syringe that I bought on Amazon comes with a safe blunt and long tip. They sell it in packs of 10 so you can share with your other artist friends. Be sure to clean the needle after each use so that you can reuse them in the future.
To remove the air channel (the part that's fused with the pen tip), you can screw it to the back of the pen's body and pull it out.
Here's a look a new ink cartridge and empty one. When it's new, there's a stopper that prevents the ink from flowing out. To use it, you have to push the air channel to the back to dislodge it. That little black thing in the empty cartridge is the stopper.
Because the opening of the Rapidograph ink cartridge is sort of narrow, if you attempt to squeeze ink from the tapered end of the Rotring ink bottle, it's going to block the opening with ink. Once the opening is blocked, it's difficult to get more ink inside.
The easiest way to refill the ink is to use the needle and syringe. The 1ml capacity syringe should fill the cartridge adequately while leaving enough space for you to put the air channel back.
It's not difficult to refill the Rapidograph. You just need the right tools.
Below are links to find the pen, ink, needle and syringe.