I believe you can import the keychain file from the old iMac:
https://discussions.apple.com/thread/7875747
> On Apr 14, 2018, at 9:27 AM, Al Poulin wrote:
>
> Using Setup Assistant, I migrated from my old iMac (24-inch, Early 2009) with
> El Capitan 10.11.6 to a new
Using Setup Assistant, I migrated from my old iMac (24-inch, Early 2009)
with El Capitan 10.11.6 to a new iMac (Retina 5K, 27-inch, 2017) initially
with Sierra and now upgraded to High Sierra 10.13.4.
All is well except for Keychain. Most Local Items did not transfer. Is this
to be