The standard "control box" no longer has depth as an option as in the Motif-XF, but there's a newer system under the touchscreen menu "Mod / Control" -> "Control Assign". Valid controllers (as sources) are "FootCtrl 1" and "FootCtrl 2" - along with Modwheel, Pitch bend, breath, and lots of other controllers.

If you set "Volume" as a destination, then you can apply any type of curve, ratio (= "slope" for some), and even edit your own user curve. The built-in curves are varied and should mostly provide a pattern that fits your needs.

The "Control Assign" curves offer an OFFSET to the default destination parameter (which for you would be perhaps Part->Volume). Some curves go from 0 to a positive number (say 64). Therefore, your foot control movement will only be additive. This is fine - set the default Volume for the part to be the minimum volume and only add. The curve polarity for only adding would be uni-polar. Note: uni-polar can also "only subtract" - but the ramp for the standard curve is "backwards" so using this would invert (loudest at heel position and softest at toe position). This is nothing that cannot be addressed with a different curve type - but I'm going to stick with the standard build-int curve. If you want to subtract and add to the default volume (meaning the default volume is somewhere in the middle and want heel to subtract but toe to add or boost) - then set the default volume somewhere in the middle - then use a bi-polar curve.

There are tutorials and articles covering this topic.

One reference: https://www.yamahasynth.com/resources/controller-box-switches