Why? I'm not exactly sure what Custom Designed meant but it sounded to me like you could store old keys encrypted with the new keys. So when migrating to HD mnemonic based key backup, you use a certain key derivation for a symmetric key and then store your old key(s) in a special event but encrypted with the new key. So when you restore from the mnemonic, the client could ask you which of your accounts to use. It could show you mentions of your legacy account even after switching to a new pubkey etc. The client could sign notifications that the legacy key was rolled over to the new key (whichever sub-key currently is active).
Discussion
If you encrypt the old key with the new key you would have to store the key + an encrypted payload. There is no way to create recreate a seed from one the keys it could generate. And because of that, you have to save both the words and the encrypted payload which given that the private key is already 32 bytes, requires more than 24 words for the two things (payload + seed) combined.
I meant you store the legacy key(s) in a nostr event. After all we trust our crypto. This event of course would be fatal if it went lost but as this could be a special kind, it might get special care on relays.
Get a new 12 words mnemonic M. Generate the master seed. Use m/12'/12' as encryption key for your kind-1212 event where legacy keys get stored.
Ohhh saving nsecs on Nostr feels dangerous.