Regarding adding entries to the popup menu, you can see how it’s implemented here. In order to add your entries to the popup menu you have to implement your own provider (see a basic example here) or override an existing one like the bpmn-js replace menu provider.
If you want to add initial properties to your elements the properties panel is not the right place for that since it only displays existing properties and lets you change those. You could simply modify the BpmnElementFactory which is responsible for creating new elements.
Hope that helps.