You may RackConnect an existing Cloud account, but any existing servers will not be RackConnected. If existing servers need to be RackConnected, they will need to be rebuilt after the RackConnect implementation has been completed. This may be done by first saving a Cloud Server image, and then using the image to rebuild the existing server a a new RackConnected server - or to simply build a new server.
Please be aware that this process for RackConnecting an existing server will result in a new IP being assigned to the server. Since you are essentially creating a new server, unfortunately this cannot be avoided.