Is it possible to modify / extend / create plugin for support SAP Cloud Platform Integration?
Do you mean in terms of deploying processes to SAP Cloud?
Is it possible to modify camunda to support different bpmn 2 model
What do you mean by that, can you be a bit more specific?
add items on toolbar
yes, this is available via client extensions. You can read about that this blog post (Extension Point for UI Plugins)
customize deployment
that’s currently not possible.You would need to implement your own deployment functionality, e.g. by a client extension