Ah cool. I thought sub accounts were deterministic from the parent. Thanks for setting me straight.
if you create an account from within the wallet then the account is dereived from the wallet-private key (cannot be exported by hand, but is part of the backupJSON file)
if you created your privkey outside the wallet .. or in a new wallet .. then there is no deterministic procedure to recover the privkey!
Read the ColdStorage wiki I wrote 3 days ago ..
BUT: on your own risk ;-)