Most OpenSSL documentation out there only shows you how to create a new key and signs it with in single command. I wanted to use a self-signed in the interm while waiting for my third-party CA approve my certificate. I already had an existing key. Here is the command to create a self-signed certificate from an existing RSA key.
#> openssl req -x509 -new -key {KEYFILE} -out {SELFSIGNEDCERT}