Export RSA Keys to stunnel.key

Question

I am trying to export some RSA Public and Private keys. Just to clarify, I am NOT trying to sign the keys. I am trying to take the raw numbers for the keys and export them to stunnel.key. My code for generating RSA Keys:

from cryptographer import RSAKey
rsa = RSAKey()
keys = (rsa.pubkey, rsa.composite)
# Export keys to stunnel.key here