There is no option for that. Your best call is to fill out a Contact Us form (you can also find it under the 'Help' tab and send it in. Or you could contact one of the admins or security members. Make sure to state what accounts you need to have merged and which account you choose to keep to save some time.
Edit: derp. I realise you mention that the account is already deleted. In that case we can’t merge it anymore. Admins can reassign posts, however. Contact an admin for that.