multiple spreadsheet files

Yes, you can use multiple Languages Sources.

There are two ways of doing that:
1- Keep I2Languages.prefab for your global terms (i.e. terms used everywhere), and add an object to your scenes with a LanguageSource component.
That is the method used in the Example scenes. The terms inside those language source will only be available in those scenes, so you can add to them, only the things you need there.
And you can have different spreadsheets for each of them.

2- Create more global sources.
By default I2Languages.perfab is the only global source. It is loaded at startup and it is available in all scenes. But you can add more sources by adding their name to the Assets\I2\Localization\Scripts\LocalizationManager.cs line 210

For example, to add the source "I2Languages_Base.prefab", modify the GlobalSources array like this:
Then, duplicate the I2Languages.prefab and rename it.
At startup, both will be loaded.

Hope that helps,
Frank