Can Material design be backported to older Android versions? With the introduction of Material Design in Android Lollipop, a consistency in app design was received quite well by the user community. However, most of these apps are showing their Material designs only when the Android version is 5.x. Is there any way to make these display the same way in Android 2.x to 4.x?

I mean, can't the design libraries be backported to older Android devices by embedding them in-app, so that the consistency is maintained across all Android versions?

I have heard about apps or more specifically, Xposed modules like Monster UI which theme your device to look like so, but System Apps aren't affected. Can't such libraries be included in pre-packaged System Apps? 

For example, a normal app uses the Holo theme in Android versions 4.x, and the Material theme in 5.x. Can't it be made to use the Material theme in older devices?
Yes is it possible. You can design your application with interactive material design using v7, v13 and v21 appcompat support libraries. These support libraries provide you backward compatibility of your material designs. For Backward compatibility read here https://developer.android.com/training/material/compatibility.html