[Kuya Jaypi]

Is it possible to add a background afterwards?
I am not sure if I just cannot find the option.

[Niels]

If you just import a png, jpg or gif with your background image, preferably in the correct size/resolution, then it will show up as a reference layer set to 50% opacity.

Make sure this layer is behind all others (drag the buttons left/right at the bottom of the x-sheet) and turn up the opacity to 100%.

That will do it!