No, the side switch buttons cannot be customized, however the buttons have preset functions built in.
While holding down Switch 1, you can switch to the small eraser tool.
Pressing Switch 2 allows you to toggle between pen tools. If the small eraser or large eraser is selected, it will switch to the pencil tool.
While holding down Switch 3,you can switch to Hand tool
The location of the switch may vary depending on the type of pen.