Is there some way to add a Slack archive like https://www.linen.dev/s/gradle-community or https://slack-chats.kotlinlang.org/ to the left sidebar of Linen where the joined communities are displayed?

Not if they are read only communities.

Sad, then take this as feature request please if possible. :-)

Can you tell me a bit more about your use case?

Well, I often go for example to the Gradle one to get a link to a conversation that does not expire after 90 days. I then need to go to the Gradle community by link explicitly. It would be nice if I could just open Linen, or switch to Linen I have open in a browser tab already and click on the Gradle one in the sidebar instead of needing to know the link or using a bookmark.