I’m not sure if you can do a soft blend through an expression pedal, but you should be able to set this up today through some remote manipulation. Maybe one button moves it to half way gain and the next moves it to full. Ie, take your highest gain profile, then dial it down to half way and then set a button on the remote, then another from halfway to clean. Then the buttons would just be in reverse of what you want. You could also get there using a overdrive effect and assign an expression pedal to the mix. Then you could setup an expression pedal for the wet / dry effect mixes from zero to 100%.
Kemper supports up to 4 expression pedals today via the remote.