Controller destinations 80 and 81 are pitch related (coarse and fine tune).  You can assign foot switch or better yet an expression pedal as the source.  This is done at the Voice level.

https://yamahasynth.com/learn/moxf-series-music-production-synthesizers/control-sets-explained-moxf/

See p.32 of the reference manual for how to use controller sets.

You select a 'Source' controller from the available options:

Source
Determines which controller is to be assigned to and used for the selected Controller Set. This controller then is used to
control the parameter set in Destination below.
Settings: PB (Pitch Bend wheel), MW (Modulation wheel), AT (Aftertouch), FC1/FC2 (Foot controller 1/2), FS (Foot Switch), RB (Ribbon
Controller), BC (Breath controller), AS1 (ASSIGN 1), AS2 (ASSIGN 2), FC2 (Foot Controller 2), AF1 (ASSIGNABLE FUNCTION
[1]), AF2 (ASSIGNABLE FUNCTION [2])
NOTE When the foot switch is set to a Control Change number of 96 or higher in the CTL ASN display of the Utility mode, the foot
switch will not be available as a “Source” of the Controller Set for the selected Voice.
Dest (Destination)
Determines the function that is controlled by the controller set in “Source.”
Settings: See the “Control List” in the “Data List” PDF document.
Depth
Determines the degree to which the Source controller affects the Destination.
Settings: -64 – +0 – +63

The 'Control List' referred to in the data list doc is on page 118.

You may also need to set the 'Controller Reset' parameter - see p.142 of the reference doc:

CtrlReset (Controller Reset)
Determines the status of the controllers (Modulation Wheel, Aftertouch, Foot Controller, Breath Controller, Knobs, etc.)
when switching between Voices. When this is set to “hold,” the controllers are kept at the current setting. When this is set
to “reset,” the controllers are reset to the default states (below).
Settings: hold, reset
If you select “reset,” the controllers will be reset to the following states/positions:
Pitch Bend Center
Modulation Wheel Minimum
Aftertouch Minimum
Foot Controller Maximum
Footswitch Off
Ribbon Controller Center
Breath Controller Maximum
Assignable Function Off
Expression Maximum

In particular note that the reset values are DIFFERENT for a foot controller than they are for a foot switch.