Example: Customer is created with entity_id of 115. Customer attribute custom_id is assigned with a prefix of "HH" followed by the entity_id. So the custom_id is "HH115". I assume this would be done with an observer and then happen on the before save of the customer account. Though, i don't know how to get access to the entity_id before the account is actually created.
First of all ,
If Magento is provide this feature then
Use Magento event customer_save_after for that case
You don't have access to the
This way you don't need a new attribute and you don't need to save anything in the db.
But if you insist on having the attribute you need to observe the event
And the method goes something like this (untested code):