We’d highly welcome contributions to Mattermost open source documentation based on the below information. If you’re interested in helping, please reply to this post?
# Slack Import
Slack Import is available from the Team Site user interface and allows people with appropriate permissions to Import a Slack team from a Slack Export file (backup highly recommended before trying this). It's designed to be a quick, end-user friendly way of importing a Slack team.
Because Slack is a SaaS product without documentation of their export format, and because they've changed the format in the past without warning, Mattermost can't ensure the correctness of Slack Import, so it's got a perpetual "beta" label to alert users of this fact.
https://docs.mattermost.com/administration/migrating.html#migrating-from-slack
# Bulk Load
Bulk Load is another option for importing Slack teams--along with users and data from other systems. It's run from the command line interface with a JSON file and is more flexibility, but requires more of an IT Admin to run the process.
There's community projects like https://github.com/Brightscout/mattermost-etl that plug-in to Mattermost Bulk Load to enable bringing in data from other systems, like Jabber. If you're an enterprise looking to import from other systems at scale you might consider speaking to the people at http://www.brightscout.com/ who have done this before.
https://docs.mattermost.com/deployment/bulk-loading-data.html