[FKHeath]

I was wondering if it were possible to add navigation to the head unit?

I've a 2018 hybrid excel and when I press the Navigation button it says something about that feature not being installed.

It would impact the car systems minimally if I could just upgrade what is already there. Maybe it's just software?

No, you cannot add navigation. There are many more components and wiring needed, such as a GPS antenna.

There are two good ways to get navigation:
1. A stand alone unit such as Garmin.
2. Use a smart phone with a navigation app. All iPhones come with Apple Maps, but I prefer Google Maps. Google Maps is free and works world wide. With a Bluetooth connection the verbal instructions will come over the car speakers. Also get a good phone mount such as ProClip, then the phone screen will be easily seen by the driver.