Hi @bitsmag,
I think this feature request is not in the scope of the process engine. The engine only allows to get a definition and update it by a new deployment. There is no modification of definitions via Api. But you can use the Model Api to modify the definition programmatically and re-deploy it.
Beside that, I think a modification Api of definitions is not a good idea in general since it could lead to incorrect definitions when the definition is changed concurrently.
Best regards,
Philipp