Is it possible to add an extra FTP user to your Marketing Cloud instance? I can't find this option within administration. We want to have multiple users due to security reasons.
Not by any standard means, as even the documentation on SFTP states the following:
Marketing Cloud allows one FTP user per MID.
Just in case you can check in with your SFMC Account Executive about this requirement, I wouldn't be surprised if they actually are able to create additional FTP accounts.
There is indeed a single FTP user / BU limit, where MID is your user name. However you can still use an external SFTP as a file location, allowing multiple folders/accounts to be added to a single BU. You can do so, by following this guide.
You need to observe that file drop triggered automations don't support external SFTP locations, but require the file to be dropped on the Enhanced FTP. This is the single key drawback of this workaround.