Add "Calculated" fields to channel

I have created a channel with companies including address and postal code. I would like to create a search field based on postal code but for that I need coordinates. I can get these coordinates from the Google Maps api, no problem.

Is it possible to create a “calculated field”, where I can add the coordinates returning from Google based on the postal code added in the entry? Do I need to create a plugin or add-on? Can I change/develop within the back-end?

I’m an experienced php developer (and getting there with Codeigniter) but new to expression engine development (I can build standard EE websites now, time for something more interesting). Don’t know exactly where to start. Hope you can give me some help! Thanks.

Cheers, Robbert