Welcome.
Yes it is very possible, it is a case of using the Mixer feature, in your Casio there are nine available individual adjustments to each of the elements.
Manuals can be and often are difficult to make any sense out of and Casio is right up there in the confusion states.
You have to be in the right mode long press PART, to get MIXER displayed and you should see KEYBOARD also displayed.
Use the cursor keys to highlight which of the 1 to 9 elements you want to adjust and then the up and down cursors to raise or lower the volume.
You will probably need to save the amended Registration to a User Bank if you want to use it in the future.
The image shows which of the 1 to 9 elements is which.
View attachment 2792