That looks like a cartridge which is supposed to be disposable. But you can get syringe and plastic type needle from daiso to fill it up. Or you can get a converter (something like a screw pump) which fits to your pen. Beware that there are many types of converters - there is standard international converter but lamy, parker, sheaffer and several others have their own converter design.

However, i think the cleanest way to do it is using a syringe or just get more cartridges. Using converter might be slightly messy because you have to dip the nib into the bottle.