I just received some great news from fellow blogger and artiste extraordinaire Marie! The Sharpie Poster Paint Markers can be refilled!!!! (I love these things!)
All you have to do is gently pull the nib off. It seems to comes out quite easily. I used a syringe to fill it with Ziller ink and then popped the nib back in. You can feel the nib 'catch' when it is seated correctly. That's it. The Ziller seems to work well and has a very similar coverage to the poster paint!
Thanks for the tip, Marie! Readers, click on Marie's name above to check out her amazing art.