Can I add Google photos to my iCloud download?

The only way I know to do that is to download them (if they aren't currently actually on your device) and then turn on iCloud Photos.

Set up and use iCloud Photos - Apple Support

I'm not sure what you mean by add to your iCloud download as Google Photos is not related to iCloud. You can install Google Photos app on your iPhone and have your photos sync to Google photos. You can also use iCloud photos at the same time. The Google Photos app is available in the App Store on your iPhone.