Geographies missing in PROD instance - Implementing Tax

Summary:

Existing Production instance has AR, AP and Procurement. Tax is being setup for AP/PO only. I found the Geographies were not imported at implementation. There are existing Suppliers and Customers. My questions are as follows:

• Business is contained to 1 state, CA, with Counties, Cities, State, and Federal Tax rates.
  a. Can these be setup manually to use for Tax calculation?
  b. What is the impact on existing suppliers? I assume the addresses would have to be cleaned up.

If we cannot use a manual setup, what tasks must be completed for existing Customers and Suppliers before importing the Geographies?