How to extend modeler stencilset

I want to add new node to the stencil set in flowable.
This node is SubProcess like and has extension elements.

<subProcess id="simulationSubProcess" >

Do you have any idea how to?
Thank you in advance.

See a recent commit (for cmmn, but idea is the same):

Quite a bit of stuff needs to be done to do it: stencilset / json converter / xml export.