It can happen. You just had a productive conversation with your co-worker, but you remember that there was still something you wanted to add. In this case it's possible to re-open the conversation up to three days after closing it.
To re-open a conversation, you have to go to the conversation history of the relevant user through the conversations overview page.
You can find the history by clicking on the new tab "history" next to "overview" or on the three vertical dots on the right side of your co-worker's conversation bar.
Inside the history you will see again three dots and the option "reopen". Clicking it will instantly re-open the conversation and you can add the information you previously forgot.
Attention: This is only possible if you're the team leader or coach in the conversation! If not, you'll have to ask them to re-open the conversation.