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I recently ran across an reference to certain Staedtler pens that readily clip into slightly modified plastic pen cases. Though I don't seem to find the on line reference at the moment, It had to do with producing PCBs, and using the ink as etch resist.

Myself, I refill the fiber tips for general output, a search for HP Plotter pens will find many vendors.
At the moment, I am working on the Kohinoor nib holders that use the smallish (about 2inch long) ink cartridges. These are ideal for fine work with the "rapid -O-graph" look.
I have a 7550a with the same pens

Hypo needles work well for refilling plastic cartridges without disassembly

Yes, I have some of those smaller Kohinoor pens. They seem to work very well on the 7575a. When using a Hypodermic, how do you do the refill? Do you make a hole with the needle? Also, how do you refill the fiber pens?

I also tried some rollerball pens, but found them very unreliable for some reason.