Yup ... I work in the business and do a lot of Chromakey jobs .... This is very easy to do! There are the equivalent of video versions of Photoshop. Every network has people on payroll to do just these types of things! With the right tools anything is possible with photos and videos ....
8 year-old kids on YouTube can have lightsaber battles, and so a millionaire running for President can add a few hundred excited crowd goers to her phony event while she lay in bed coughing up bloody sputum