How can I use an existing private key to a new iOS development certificate?

For aesthetic reasons, I would like to use the same private key that I used to create my distribution certificate a while ago, to create a new development certificate (my old one expired). But the "How to create a development certificate:" on the iOS provisioning portal require that you use Keychain Access create a new key. When I try to do Keychain Access > Certificate Assistant > Request a Certificate From a Certificate Authority With "(my key name)" the Certificate Assistant doesn't actually create a CSR file.

Does anyone know if this can be done, and if so, how?