Adding TXL would not theoretically be a problem so long as you don't exceed stopover (destination + 1 stopover) and segment (4 in each direction) limitations. You could avoid any backtracking issue (which really isn't a problem, though it may nonetheless bother some agents) by routing TXL-MUC/ZRH-US.

But your overall routing is technically illegal now because it crosses both oceans (a recent tightening of the routing rules). You may therefore run into difficulties making any changes.

So you can try, and it might work, but don't be surprised if you spend lots of time on the phone accomplishing nothing.