...so is it a case of assigning the pedals to the Kemper along with the expression rather than trying to go through the front of the amp?
Yes, you could try putting the fx pedals in the the kemper's loop. That would let you have a Kemper wah effect (controlled by your expression pedal) before the chorus and distortion, which is where it would usually be. You would also be plugging your guitar into the Kemper.
That shouldn't be a problem unless you want to use your Soldano's preamp. In that case you could try looking up the four-cable method, and see if it can be used with the Kemper, your amp and your effects.