Auto tag posts by importing a Spreadsheet (CSV)?

I’d like to find a way to tag already existing posts by importing a spreadsheet full of post names and tags.

Let me give you an example:
The website I’m creating has a post setup for each of a dozen or so offices within a company. Each office has a catchment are which is defined by postcodes / zip codes.
The postcodes are setup in wordpress using a custom taxonomy, with each tag within the taxonomy being a postcode.

This relationship between each office and the postcodes that ‘belong’ to it are kept by my client in a spreadsheet. Every couple of weeks new office is added, which changes all of the postcode catchment areas and therefore all of the ‘office’ posts within the website need their tags updating to reflect the new catchment areas.

Rather than a massively tedious manual task of re-tagging each office with 20 or so tags each time a new office is created I would really like to find a way where my client could upload their updated spreadsheet into wordpress which would automatically re-define the relationship between these offices and the postcode tags that belong to each.

Is this possible? I’ve been searching the web and looking through various plug-ins but haven’t found anything that does it yet.

Many thanks
Stef