Source Audio Soundblox2 OFD & Multiwave Bass Distortion with their expression pedal can do it. With two presets you could programme either two different types of distortion that can fade in or out depending on how you select the exp option,
Or if you just find one killer sound, set the exp option to fade in (more gain) on toe down on left button & the opposite on right button ... quite a few options really as you can select the exp assign to any parameter, use as volume pedal or set to clean blend or maybe low to high eq (treble or mid or bass) etc ....
You can also use 2nd out on exp pedal to send signal to any pedal that has 1/4" trs exp input and still send to SA pedals (they link via 1/8" trrs cable) .. very versatile