I'm making a machine that needs the feedrate of axis X to increase proportionally to the position of the Y axis. Is this possible? Can this be done with acceleration in the motor tuning configuration?

YOu could make a brain that monitors and compares the Y position then alters the Feedrate Override based on the % of change of the Yposition.

BUT remember it will alter the feedrate on ALL axis not just X.

Just a thought, (;-) TP

Depending on how smooth running it has to be you can use a macro that does this for you in fast steps.
output:
===================
y_step            = starting_point
y_limit             = ending_point
y_step_change = delta_y_movement

y_increment = 0

while (y_increment < y_limit)
    x_step = (y_related_formula)

    code "GO Y" & y_step       ' desired Y step
    code "GO Z" & x_step       ' desired X step
    While IsMoving()            ' Wait for stop
    Wend
   code "G4 P" &(pause_that makes_Y_go_at_your_desired_speed)  ' "G4P0.25" pauses a 1/4 sec
y_increment = y_increment  + y_step_change
wend
===================

make sure the Y_step is plenty small to accommodate the increasing speed of the X axis and the material you are working
This will move y linearly and x according to your formula.
Both the step size and the delay will affect the apparent feed rate

Thanks,
Bill