How can I have "add to keychain" default to being checked?

In other words, how can you make your Mac less secure?

I don't think there is a way, unless there's a plist file or a terminal command that will let you do it.

Not to be critical, just a concerned member, the Keychain is a very bad place to store your passwords. It defaults to unlocked in administrator accounts and uses a broken encryption scheme, which means it wouldn't be hard to crack it. I recommend using KeePassX (free) or 1Password ($60, but far more convenient). Both are very secure and will do a much better job of protecting your passwords than Keychain Access will.