How to create custom type element?

I want to create a custom type element.
But I want to apply all the functions registered in the existing ‘bpmn: Task’ element to the same element.
I want to change only the type, for example ‘custom: legacyTask’ like this …

What is the best approach?

You’d have to extend the BPMN model in order for bpmn-js to understand your custom type. Have you looked into this example?