Normally when copying Keychain items from one to the other, the Keychain Access program will ask you for your password for each and every item. Here's a time saving workaround.
First, have the destination keychain (which may be the login keychain) and the source keychain, which you want to merge the items from, both opened up in /Applications/Utilities/Keychain Access.
Now change the password of the destination keychain to be empty. You can do this by selecting the keychain, going to the Edit menu, and choosing the item to change the password. You will need the current password (probably your login password) to change this.
The Keychain Access program will probably complain that the new empty password is not secure enough; but you can override this by holding the Option key and clicking OK.
Next, select all items you want to copy to your destination keychain and drag them over. Since there is no password set, you can just click allow for each item. Huge time saver.
[crarko adds: I tested this, and it works as described. Since it is the destination keychain that you removed the password from, remember to reset it (back to whatever you had to type in to edit the password) when you are through.]
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