Hi @egoitz! Thank you for the additional information.
There is a setting here that might help: https://docs.mattermost.com/administration/config-settings.html#enable-users-to-open-direct-message-channels-with.
We are currently also exploring other alternatives, such as single and multi-channel guest accounts: https://mattermost.uservoice.com/forums/306457-general/suggestions/9982719-ee-single-channel-guests-for-non-team-members.
Other than that, I believe we don’t currently have support for this specific feature. If you’d like, you can contribute this in the feature idea forum. If you do so please include a link back to this Forum issue.
Let me know any questions!