[Jesse Toula]

Ok, I’m a little bit confused about what your problem is. Are you having trouble tracking the footage? I thought you had tracked it on to three nulls.

Or are you trying to attach the extension to the track?
To fix that you would parent the three nulls to each other so that one of the nulls contains the entire shot’s movement. (explained in the tutorial linked in my previous post)

Or are you have a problem with the mask not tracking with the footage?
The problem here would be that the mask is not going to stay with your footage unless it is roto-scoped. What you would want to do is use a matte. Copy your mask to a solid, then parent the tracked null to it. Then use the solid as a matte for your footage, that way the transparency you created stays with your clip as the extension does.

Let me know if any of these is what your problem is and I can go into more detail if you need it.

Jesse Toula
https://www.BatchFrame.com

[Jesse Toula]

Have you checked out this tutorial. It talks about exactly what you are doing. (as far as I can tell)

https://library.creativecow.net/articles/kramer_andrew/Set_Extensions.php

-Jesse Toula
https://www.BatchFrame.com