We’re working on a tool that will let you import users, teams, channels, and posts in bulk.
The outline for what’s planned can be found here: https://docs.google.com/document/d/1yo1ifh2Q195ejaTW0IAFvqtheT7h9bSHCzwETBhIiic/edit
If it’s something you’re interested in, please feel free to add comments or come sign up to our team site and join the Bulk Loading Channel to come discuss the topic and follow along on progress.