I’ve found two extensions that sort of handle this situation, but neither look to be a perfect fit.
Adds a new ‘type’ attribute to customers - ‘customer’ or ‘contact’
‘Customer’ types can have multiple associated contacts.
Adds ‘sub-logins’ to customers. A ‘sub-login’ has name, pwd, & email fields only.
Unfortunately, I have not been able to try either of them :(