Is there really no way to get all mails from different accounts in one inbox and also keep them separate in own inboxes as it is e.g. in Mailspring?
Sounds crazy to me and, again, THIS IS NOT A ROUND CUBE ISSUE.
However, sure, there are surely many dozens of different ways to do that, all of which have to do with your MTA / SMTP server scenario.
As you appear to be on a variant of Unix called Linux, you could use the .forward mechanism. Or, you could use a SpamAssassin-like daemon called by your mail server. Or, you could use ProcMail, or...
Once you have the emails into a single mailbox, just create a link (via ln -s) in each user's mailbox subdirectory pointing to the merged mailbox - it should show up "automagically" if your IMAP configuration is set to automatically provide "subscribed folders" (as some systems call 'em.)