Can I add WidgetKit extensions to an app with target iOS < 14?

Yes it is possible! WidgetKit is only an extension added to your application that would render on devices which support that particular extension. You can definitely create an app that would run on devices <= or > iOS14, iPadOS14, macOS11 and so on and the widget for the same which would be rendered on devices >= iOS14, iPadOS14, macOS11. WidgetKit is just an added facility to your application just like there are certain features of an application that can run on higher devices but not on old versioned devices.

Yes, you can as I develop the Widgets on an iOS13 Xcode project. You can see them on an iPhone that runs iOS14 and not on the iOS13 one. You just need to add @available(iOS14) in your code because you are using WidgetKit. But don't worry, Xcode will remind it to you nicely. This is the WidgetKit framework that is not available under iOS14, iPadOS14 or macOS11.

Select iOS 14 for your new target