I've never seen an option to assign a due date to an expense report in the system. You may have a policy that states reports are due let's say by the 28th of the month, but how would you set that up in SAP Concur? It is more of a policy your company can have, but not a policy or rule you can put in place in the SAP Concur system.
Maybe you can provide a little more information, but I'm 99% sure this isn't possible. I say 99% because there is a 1% chance there is some secret way to do this I don't know about. :-)
Many thanks for you answer.
After research, I'm 99% sure this isn't possible. I asked here thinking that maybe there is that 1% ) which I did not find.