Previously you’ve been using extension elements, which is fine. Creating something like <bpmn:MobileTask> is technically possible from our toolkit’s perspective. However, you won’t be able to open such a diagram in any bpmn-js instance that isn’t modified, let alone any other editor. The diagram will also not be executable in any engine that isn’t modified. I’d like to understand why you need a custom element instead of using extension elements. If it’s only about the appearance and a few properties you can easily do that while ensuring BPMN 2.0 compatibility. I’ve created a simple example that shows what I mean:
Let me know if this example is helpful.