Well there is, but I wouldn't recommend it. The white square right next to the bar 1 marker can be dragged backward to start the song at a negative bar number. It gets kind of buggy though. For example, the code for copying and pasting automation relies on the song starting at bar 1 and it can get a bit weird if the song starts elsewhere. I'd recommend just starting at bar 2, or better yet, bar 5 (because then you skip a whole measure and your measure markers will still line up).