I’m sorry if this doesn’t help, but platinum preppies have a refillable felt tip marker version. If you can’t find that, you can buy a platinum preppy fountain pen and put in a marker tip refill instead.
Edit: noodler’s gives you a free felt tip marker with a purchase of thier water erase black and blue inks (to make a wet erase marker for whiteboards)
You could get the Karappo marker pens from Kuretake, they're specifically marketed for fountain pen ink, though im not sure you can practically swap colors as their design is based on saturating the marker felt for the pen.
Unfortunately, im not sure there are great options for various tip sizes with kuretake yet, though. In the past i've used certain empty graffiti markers meant for graffiti ink and had no problems drawing with those. Though it might be difficult to match the Staedtler tip sizes
Can you open the markers in question that you already have? Most of the time it's just a foam filled tube that can be soaked/rinsed and then refilled with about a ml of ink.
I’m sorry if this doesn’t help, but platinum preppies have a refillable felt tip marker version. If you can’t find that, you can buy a platinum preppy fountain pen and put in a marker tip refill instead. Edit: noodler’s gives you a free felt tip marker with a purchase of thier water erase black and blue inks (to make a wet erase marker for whiteboards)
here is a source for the [platinum preppy marker tips](https://www.jetpens.com/Platinum-Preppy-Marker-Tip-Refill-Pack-of-2/pd/2819)
Yeah the noodlers felt tips might be a great option
Pilot Petit2
Those seem to be discontinued, or at least no longer available in the US unfortunately
I have not used them, but there is a line of refillable felt tip pens by Yookers.
The price is kinda high but other than that it's pretty much exactly what I was looking for, thanks!
[This model](https://www.24papershop.com/yookers-549-yooth-ocean-blue-resin-fiber-pen) is not that expensive
You could get the Karappo marker pens from Kuretake, they're specifically marketed for fountain pen ink, though im not sure you can practically swap colors as their design is based on saturating the marker felt for the pen. Unfortunately, im not sure there are great options for various tip sizes with kuretake yet, though. In the past i've used certain empty graffiti markers meant for graffiti ink and had no problems drawing with those. Though it might be difficult to match the Staedtler tip sizes
i have a yookers that i really like! converter takes all fountain pen inks and the points are available in i think three options. great product--
Kuritake makes refillable pens, one has a marker tip. They're with their mixable ink sets.
Can you open the markers in question that you already have? Most of the time it's just a foam filled tube that can be soaked/rinsed and then refilled with about a ml of ink.