Translate Zip Code to City field

I was wondering whether at the time of registration and only then, as I'm having new subscribers provide their Zip Code, I could have some script translate the zipcode they write through a database that I can install, into a city name and have that name show up in the city field?

If you already have a database established where you can translate the zip to city then yes. You could do this using CB Queries, which allows executing database queries at various CB triggers such as after registration. If something more complex was needed you could use CB Code to execute a PHP file that handles all the queries, connections, etc.. in that could could probably even use Google Maps API using CURL requests. Then perform a database query to insert the city into the users CB table row.