Hello! Yes, it is possible to do this. In FaceRig, click on the UI+ button to switch to Advanced UI, then click on Load Video as Input Source and browse for the video (you can only use .avi files): the avatar will move and track the movement of the character in the video.

However, if you want to add an avatar to a video in the sense that you want to create a video where there's an avatar moving and talking next to a human or any other visual (say, as if you were creating a movie), that can be achieved by using a .webm file as a background:

To put a .webm video file as your background, first you have to open a notepad, then type in:

<video width="400" autoplay loop>
<source src="file:///path of the file\name of video.webm" type="video/webm">
</video>

"400" represents the size of the video and, if you want it full screen, just type 1280 instead.

Once you've done that, right-click on "File" (in Notepad) and then on "Save As" and name your file with the .html extension.Open the new .html file and then copy-paste the address found on the web-page that will be launched in the field found in Customization -> Backdrop -> Custom Web Page.

Then, while the video in the background is playing, you can reposition the avatar in the scene and record.

Or, of course, you can use 3rd party apps to edit and create your video.