Merge two backups TimeMachine from two different Macs

Hi,

I've done some research about this question in this forum but was not able to find an answer to my particular problem. Here is what I'd need to know:

1. I had an iMac which has been backuped by TimeMachine for several years
2. I replaced it by a new MacMini on which I copied the latest TM backup from the iMac in order to have the same configuration.
3. I reconnected the TM hard drive to the MacMini.
4. Unfortunately I cannot access anymore my former backups of the iMac and the TM on the MacMini started with a new backup on the same hard drive which is now to small.
5. On the TM hard drive, in the Backups.backupdb folder there are 2 folders now, iMac and MacMini

My questions:

1. Is there a way to merge those 2 backups? I'd like to have one single backup so I could access all my former backups thru TM.
2. If there is no way (even not with some Terminal commands) I'd like at least to move my old backup to another hard disk (for archive). I've already tried it (copy to my main Mac hard drive which is in APFS format), but I get this message:
3. I've also tried to compress the iMac TM-folder but I get this message:

Any ideas?

Thanks a lot!

Christoph