(Disclosure: I work for Microsoft, but not as a member of the Microsoft Teams group)
I've released a new version of my Microsoft Teams command line tool which I've called Channel Surf. The existing version lets you re-create your existing Slack channel structure in Teams and bulk add new Teams channel.
Archive Slack messages into HTML files and store them in Teams
Copy Slack file attachments into Teams
Further details are up on my GitHub page.
How? Slack enables a team owner or admin to download an archive of a their team's history, which Channel Surf scans and then uses in conjunction with the Microsoft Graph API to re-create Slack channels within Microsoft Teams, generate HTML message archive files, and upload these archives and associated file attachments.
It's a work-in-progress - I've only tested this on a couple of small Slack archives. The display of user names, date and time stamps, message formatting, and viewing experience will be improved as I have free time. But for those of you wanting/needing more - there's always the fork and clone buttons to help you get started.
As a gentle reminder, I'm reliant upon public Microsoft Graph and Teams API's. The API's needed to perform a true message import aren't yet available: these include impersonating a user, backdating the creation date of a message, and probably a few other things I haven't thought of.
Hopefully for now, some of you will find this useful in your pursuits.
Is there a limit to how many HTML message history files will be created? I noticed in the command prompt for one particular channel that all message history json files were created in Temp but only 18 were uploaded to MS Teams.