I have a similar question. Is it possible to create a subaccount in a wallet that does not contain the "top-level" account? Could I create and register a subaccount and then move it using riverhead's step 1 and 2?
Cannot do it today, but I would like to give the ability to hide and/or group accounts.
AFAIK what svk asks for should work ...
1) go to your empty wallet ..
2) create you subaccount foo.bar
3) get the account pubkey for it
4) got to the wallet that contains the mother account
5) add a local contact for the subaccount with the corresponding pubkey/name .. foo.bar
6) register the local contact
however .. you will not be able to "recreate" the subaccount's private key from the mother account's private key ..
TL;DR; .. it's 'messy'