Q: "How can I assign a record to an inactive user?"

A: (1) You need to have salesforce.com Support activate the "Create Audit Fields" permission for you (and this is typically only activated temporarily), (2) this can only be done through the API (with a tool like Data Loader), and (3) it can't be done for all objects.  But it is indeed possible!  For more details, see http://tinyurl.com/AssigningInactiveOwners