That's not likely to work I'm afraid. The default value is just the value that is used if no other value has been specified. Whilst it might be theoretically possible to lookup that value from another location, not only would that be somewhat wasteful of resources (requiring a separate database lookup for each field), it would only work the first time the field was used. After that, the value that was pre-populated would be persisted in the database, and even if the person updated their profile in the other component, it would have no effect on nBill which would continue to use the original value.
Unless the components involved include mapping features specifically designed for the purpose (which nBill doesn't), synchronising the profile fields of 2 different components is not trivial and can only really be acheived by a programmer with a reasonable knowledge of PHP and database interaction, and even then, only if the component that is being used as the master provides some way of hooking in to profile update events (nBill does).