CanvasSize

app.command.CanvasSize {
  ui=true,
  left=0,
  top=0,
  right=0,
  bottom=0,
  bounds=Rectangle,
  trimOutside=false
}

ui: false if you don't need to show the dialog to the user.
left/top/right/bottom: How many pixels to add, or remove if the value is negative, to/from left/top/right/bottom sides.
bounds: A new rectangle with the new bounds of the canvas. If you use this argument, left/top/right/bottom are ignored.
trimOutside: True if you want to delete all the pixel content of cels that will be outside the canvas after the canvas size is modified.

Examples. Add 8 pixels at the left side:

app.command.CanvasSize { ui=false, left=8 }

Add 8 pixels to the left and 8 pixels to the right (new width = old width + 16 pixels):

local r = app.sprite.bounds
r.x = r.x-8
r.width = r.width+16
app.command.CanvasSize { ui=false, bounds=r }