Maybe this is of use:
https://git.scytec.de/danthefan/slack2mattermost-dms-converter

Please! We have a 8 MB Export file with 70.000 Messages from the Slack Corporate Exporter.
Could you giv us an hint how to code an individual .json in order to import at least the most important 3 channels and/or DM-History?!
Thanks!

Please implement the import of Slack private channels that are contained in an export generated by the Slack Corporate Export tool so we can migrate to Mattermost.

This is the data of the slack-corporate-export format:

├── channels.json
├── subfolder_channel_1 (i.e. "General", "public Information")
│ ├── 2019-12-20.json
│ ├── 2019-12-21.json
│ └── ...
├── ... (more subfolders)
├── groups.json
├── subfolder_group_1 (i.e. "secret Group", "Management")
│ ├── 2019-12-20.json
│ ├── 2019-12-21.json
│ └── ...
├── ... (more subfolders)
├── mpims.json
├── subfolder_multi_person_chat_1 (i.e. "mpdm-joe-lisa-anna-john", "mpdm-lisa-john-peter")
│ ├── 2019-12-20.json
│ ├── 2019-12-21.json
│ └── ...
├── ... (more subfolders)
├── dms.json
├── subfolder_direct_messages_1 (i.e. "D38RUJBCP", "D39AJJG7L")
│ ├── 2019-12-20.json
│ ├── 2019-12-21.json
│ └── ...
├── ... (more subfolders)

the json of the direct messages (dms.json) look like this:

[
{
"id": "D38RUJBCP",
"created": 1529308826,
"members": [
"U63MAB7U5",
"U1D4CHJLV"
]
},
{
"id": "D39AJJG7L",
"created": 1527578772,
"members": [
"U63MAB7U5",
"U1GKUUV7H"
]
},...
]

It would be great to get some information how this feature is scheduled, so one could plan when it will be possible to move over to mattermost :)

Tool that some are trying to use: https://get.slack.help/hc/en-us/articles/201658943-Export-your-workspace-data#use-corporate-export

Hello Andy- wondering if you had any luck with the documentation or if you could provide us the format of your export so we can better understand how we could support this request.

If any other community member could provide an example format of the export, it would also be appreciated so we can analyze this request further.

Hello Andy. Can you provide me some more information about what format the export is in? We do have a bulk import tool (https://docs.mattermost.com/deployment/bulk-loading.html) that can be used to help import your data. There may be a way to convert the format of your Slack export into a format that will work with this tool so that your data is retained.

Hey Andy! It is planned but not yet scheduled. I'll forward your feedback to our product team!

Any update here? Is this on the roadmap?

I am importing from a paid slack instance (enterprise export) into an open source version for testing, but our main mm server is enterprise edition.

Looking into this now. Are you importing from a paid Slack instance to an open source version of Mattermost or Mattermost Enterprise Edition?

Our employees are quite resentful of our switch to MM because they've lost their DM and private channel history !