Hi Alexander,
As far as I know this isn't possible at the moment.
I'm basing my reply on this question that I asked some time ago.
It's not quite the same question, but the reply indicates the current integration is somewhat limited, and the developers are keen to get ideas about how to improve it.
Thinking about it "on the fly" so to speak, to add your own custom field types to the builder addon would mean modifying the code of the addon itself, and I'm not aware of any hooks that allow for this right now.
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