If you wish to edit your background/plate layer from a moving image, you are able to take a still image of one frame which can be edited and then brought back into AE to add to your background layer.
In this example we are going to create an extension of the side walls.
In AE, solo the background layer by clicking on the Solo dot for that layer.
Place the playhead at the end of the layer.
Go to Composition > Save Frame As > File
A window appears asking what format you would like to save the file as, click on OK & render/export the file
Open the new file in Photoshop.
Duplicate the Background layer.
Use the Rectangular Marquee tool to select part of the original wall, copy and paste this to create duplicates of the wall across to the edge of the frame.
The new layers can be merged into one layer by highlighting all the new layers, selecting Layer > Merge Layers
Use the Clone stamp tool to edit the wall and tidy up any obvious marks/lines.
The left hand side wall can be created from a duplicate of the right side extension, and again use the Clone stamp tool to tidy up any patterns/shadows etc.
Also, the benches in this image would need covering/removing. Try using the Content Aware Fill or the Clone Stamp tool.
Written by LA