How to refill cartridges
The cartridges are very comfortable and practical but have a couple of fundamental defects: they are disposable, and therefore not environmentally sustainable, and can only be used with the colour of ink with which the supplier has filled them. For this reason, a good choice, the only one possible for pens with cartridges whose format no longer exists, is to fill them once they are exhausted.
The concept is very simple, as is its realization. All you need is a syringe (better if it has a very wide needle, to avoid deposits) to load the ink from a bottle, and then pour it into the empty cartridge by inserting the needle into its nozzle. Obviously a minimum of practice is necessary and and it is not advisable to exaggerate when filling the syringe, to avoid inserting into the cartridge more ink than it can contain. For an example of the procedure you can see this video.