You can export data from HipChat Data Center (see https://confluence.atlassian.com/hipchatdc3/export-data-from-hipchat-data-center-913476832.html) and use the Mattermost ETL framework to import the solution (see https://github.com/Brightscout/mattermost-etl).
Learn more about migration options here: https://docs.mattermost.com/administration/migrating.html#migrating-from-hipchat-server-and-hipchat-data-center-to-mattermost
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would defo want this - using hipchat with a <10 person team, want to get all devs and IT onto a single platform ~ 50 person tech chat team. would like to move our existing hipchat history to new platform.