I tried refilling a rollerball pen with fountain pen ink. Since I couldn't get the feed mechanism out, only the core part (that looks like a small ball point pen refill) I used a syringe to inject the fountain pen ink into it. It worked, except now the feed mechanism was completely clogged with ink. I also had to force the core back in, again, and it worked, but only once, it won't take that abuse a secod time.
So how to get the ink out of the feed mechanism?
Blood sample centrifuge. Only took a couple of seconds, I didn't even have to let it spin up all the way.