There are instances when the user would like to ensure that the their customers under go all three stages of being a customer, from lead to prospect to customer.
Since there is an option via Lists > Relationships to add new prospect and/or new customer.
As an alternate solution, the user can create a workflow such as below:
Record Type: Customer
Sub Type: Customer , Prospect
Release Status: Released
Enable Logging: checked
Event: On Create
Trigger Type: Before Record Load
Action: Return User Error
Trigger On : Entry
Parameter > Text : <desired text>