both cartridges and converters have the same issue which is that they are designed to go into the barrel of every model of pen made by the manufacturer.
With the caveat that the expansion space (hidden from view between the reservoir and the nib) may not tolerate a larger ink reservoir in your pen (NB your range of temperature changes makes a difference too), it may be possible to fit a much larger ink reservoir. If the pen has a metal barrel, you may find that there is a plastic insert (which normally pushes on the top of a cartridge) that needs to be drilled/removed before you can install a larger reservoir, (or indeed any converter in some models).
The new reservoir can be made using the base of a cartridge, and a length of plastic tube, glued (or welded) together. The other end of the tube can be plugged permanently, or with a removable stopper of some kind (which can be retained by the barrel once it is screwed home). In the latter case the reservoir can be refilled using a syringe or w.h.y.
If you want to make a cheap refillable reservoir and you are happy to use a syringe, you can drill (and tap) the top of a cartridge to accept a small machine screw, or some other kind of plug.
Regarding inks, someone has mentioned clogging with some inks. This is certainly a risk. However there is another issue, which is corrosion; not all inks are equal in this regard. Since ruining an expensive pen with another brand of ink, I now stick to Quink (which is available in red) where possible since this doesn't seem to corrode pens too badly, and allows relatively easy clean-up. Maybe there are other inks that are similarly benign in this respect...?
cheers