Thanks for the quick reply @lfbrock. We are mainly looking to migrate our active conversations from Slack to Mattermost in order to continue work. Ideally, we would like to be able to map accounts in Slack to accounts we already have in Gitlab since the email addresses are exactly the same. So an option for merging accounts would be preferable. It also looks like the import is preventing users from logging in due to them containing the same email address. So even with an invite link it would say the following:
Galois Mattermost needs your help:
Team example-team already has a user with the email address attached to your gitlab account
So if we could do some sort of account merge, then that would be best. I can see use cases where changing the username from Slack would be appropriate as well, so I think giving the option as opposed to picking only one would make this a little more flexible.