OSX ssh-agent: no password pasting, and problem with PKCS#8?

I use ssh on my machine, and have set up a long not-human-friendly passphrase which is saved in my password manager. What makes me crazy every time is that I cannot paste into the window pictured below. I know the Remember password in my keychain option and use it. Sometimes I have to enter a new one though. Why can't I paste into a password field?

Same thing for the dialog which pops up when plugging in encrypted disks.

Some updates:

1. I found out how i can circumvent the dialog: just use ssh-add -K ~/.ssh/id_rsa, and then I can paste the passphrase into the terminal.
2. As mattmcmanus mentioned, my Key is encryted using pkcs8 which was broken on Yosemite's ssh-agent. This is unrelated to the above problem, but it did occur to me at the same time (read here).
3. using ssh-add still not fixes this for encrypted disks.

1. The dialog for ssh-agent can be circumvented by adding the key in the console/terminal: ssh-add ~/.ssh/id_rsa. You can then paste the password into the terminal. Also, adding the -K option to ssh-add will save it to the keychain as per Oliver Lacans comment.
2. As mattmcmanus said, id_rsa encryption using PKCS seems to be broken on OSX Mavericks ssh-agent. The fast workaround is to decrypt the keyfile, and encrypt it again with the standard ssh procedure (Key-derivation method: MD5...):

mv id_rsa id_rsa.pkcs
openssl rsa -in id_rsa.pkcs -out id_rsa
# enter passphrase to decrypt
chmod 0600 id_rsa
ssh-keygen -f id_rsa -p
# enter passphrase to encrypt again

Is it a PKCS#8 key and are you on mavericks? If so, that seems to be completely busted. Not sure there is a solution yet.

On Mac OS X El Capitan the ssh-add command can do this with a -K option:

$ ssh-add -K ~/.ssh/id_rsa
Enter passphrase for /Users/your.username/.ssh/id_rsa:

You can paste your password into this prompt with ⌘-V.

A session for this looks like this:

$ ssh-add -K ~/.ssh/id_rsa
Enter passphrase for /Users/your.username/.ssh/id_rsa:
Passphrase stored in keychain: /Users/your.username/.ssh/id_rsa
Identity added: /Users/your.username/.ssh/id_rsa (/Users/your.username/.ssh/id_rsa)

Relevant excerpt of SSH-ADD(1) brought up with man ssh-add:

-K When adding identities, each passphrase will also be stored in your keychain. When removing identities with -d, each passphrase will be removed from your keychain.

INFOSEC NOTE: Your password won't be shown in the shell, but will be exposed to all running applications via the clipboard. An autotype solution would be more secure.