To add the key, you need to convert the key format from the Privacy-Enhanced Mail (PEM)-encoded format that SSH uses to an OpenPGP-formatted certificate. The Monkeysphere Project provides a utility, pem2openpgp, that does this for you.
Unfortunately, making this newly added key a subkey is not a one-step process. This longer process is required because there is no clean way to delete the GPG key in the keyring that is just the SSH key. The keys are identified and operated on by keygrip, and the keygrip for a key is the same whether it is a subkey or a standalone key. Thankfully, you only need to work with the private keys, as you can regenerate the public keys at the end.
Also, if you have a newer style OpenSSH key, you'll have a couple of extra steps to convert that into something pem2openpgp can read. Unfortunately, as of version 0.41, Monkeysphere cannot read newer style OpenSSH keys. (Your key is a newer style key if the first line of the private key file is: `-----BEGI