keychain add file

I recently upgraded from my old iMac running Sierra to a new iMac running Big Sur. I want to copy my old keychain to the new machine. I tried various methods. The most successful is to copy the file from the old library and use "Add file" on the new one. It works, placing the new keychain into the section "Custom Keychains" and I can access it. However, when I quit Keychain Access and restart it, the new keychain has been moved into the section "System Keychains" and the contents are gone. I cannot delete it.

The only way to visit my keychains is to change the name of the keychain file and reload it. I can use it until I quit Keychain Access. I now have five blank files each with a slightly different name under "System Keychains". I have appended "001" to the name of the keychain I copy and update it by one each time I need to reload it. The copied keychain says it is locked and offers the option to unlock it but nothing happens when I try. I can make changes to the keychain, except, of course, the changes disappear when I quit Keychain Access.

Any help would be appreciated.