Its hard to tell based on only pictures, but I'll take a shot.
I don't know your skill level so bear that in mind. Also, if the desk is made of particle board and just has those cam lock screws holding it together, it wont be real easy to assemble in a manner different than designed.
Looking at the diagram of the center piece, it looks like the center just sits in there and that the long inner leg panels are attached to only the right and left sides.
You would probably want to remove the inner leg panel from the left side and somehow attach it to the center piece of desktop, probably close to the end so it doesn't leave an unsupported section. If you do that you definitely have to use the file cabinet to hold up the left side.