I just got my first Omas pen with the funny "Advanced" Cartridge System. I don't like the cartridge only system and I want to use my bottle inks. However this is not easy and I googled around for a while without finding a satisfying answer. So I decided to try something myself and actually find a "solution" I want to share with you guys:
You only need three things which I think most of us can find pretty easily:
1. A short international converter. There are a couple of them around and I think any of them would be fine. The main criteria is that the ink reservoir part of the converter should be shorter than a short international cartridge so it fits into the upper half of the metal basket. This way, the bump at the middle of the basket will not be a problem
2. Some sort of tube that fits into the end of the basket. One easy choice, which is what I chose, is a short international cartridge with the top part cut down.
3. Some spring that fits onto the handle of your converter and about the same diameter as your tube (international cartridge).
With these three things, you simply put the spring onto the handle and put the lower part of the handle into your tube. To install the converter into the basket, you first put the tube end into the basket and then compress the spring by pulling the converter until you can fit the converter into the basket. The spring will spring back and make sure everything stays in place.
I'm quite satisfied with this solution for now and hopefully this will be helpful for some of you.