Hi. Found this hint in some other support sites, but haven't yet tried it myself. At least 3 posters confirmed it works.
If you have both old and new KeyChains open by Keychain-Access utility in front of you, you can basically copy and paste items from one to the other.
HOWEVER…
You will have to authenticate for each item separately… which is a very annoying at best, and impractical if you happen to have hundreds of keychain items in your old login keychain.
The trick is --- to set the password to nothing on the destination keychain (the new login keychain created on your new Mac). You set a keychain password (for the new keychain) in the Edit menu of Keychain-Access. It will complain, and even refuse to set an empty password - and you need to Option-click the "OK" button to force it accept an empty password.
Now, when you copy and paste items from old to new keychain, it still opens a dialog, but you only need to click "OK" without authenticating (typing your password) for each item. Saves lots of time, although lots of clicks to go through.
Then --- DO NOT FORGET --- to set the new login's password to the original one, not to lose your security.
I just received my new 27" retina iMac, beautiful and new - I could not use Migration-Assistant to move over my account (my older Mac is just way too old, and contains LOTS of junk back from 1985 !!!) so I wanted to move things one at a time, and got the same situation you are.
I'll add my own feedback on this method tomorrow, when I'm finished.