@Arno - if you have already configured the slot as type: list, you should simply be able to annotate your training examples in a very straightforward way.
Let’s say the slot that you’re using is topicList. I would annotate your example as -
I would like to subscribe to [topic_1](topicList) and [topic_2](topicList)
If you do so, you’ll get a single list object called topicList that contains both ‘topic_1’ and ‘topic_2’.
topicList:['topic_1', 'topic_2']
In effect, you can use more training examples to let the entity recognition itself add the slots to a list.
To remove a list item from a list however, you would need to run a custom action after you identify the right intent. So if the user says -
Could you replace topic_2 with topic_3?
You can annotate it under a new intent as “replace_topic” as
Could you replace [topic_2](topicToBeRemoved) with [topic_3](topicList)?
You can then use a custom action after replace_topic to remove topicToBeRemoved from topicList.
Does this help?