Adding a value to a customer ID?

Hello!
I have a case number for a patient. 2009-001, for example, in C2.
I also have a contact number for the patient's family member, in D2.
How can I combine the patient's case number 2009-001 from C2 with an entry
in D2, 2009-001-01for each contact?

Example:
Jane Doe is case number 2009-001. She has two children, Josh and Jolene.
Josh would be contact number 2009-001-01. Jolene would be contact number
2009-001-02.
Then we'd have another case number: Doctor Pepper, 2010-003.
She has three contacts: Coke, 2010-003-01, Sprite, 2010-003-02, and Jack
Daniels, 2010-003-03.

How can I add the "-01", "-02" and "-03" automatically?
Thanks