Hi @amilak
You could implement a task listener that just sets a variable. That could be done by using Script. Then you can include an event sub process into your process. Everytime the variable turns true the event subprocess is triggered. (You need to make sure to set it false again then) Inside the event subprocess you can then use a service task which contains the logic of the Listener. This can be then again an external task.
This forum post follows a similar approach: Can we execute external task on task lifecycle events