I have bad news and good news.
The bad news: Changing a canvas size, after a drawing has begun, isn't possible in the current version of Draw. It's also not possible to move the layers (either as a group or individually).
The good news: It will be possible in the next version of the app (which should be available in a few weeks.
I know none of that helps you now... Unfortunately, right now, the only way you can get your outside-the-canvas strokes into the composition is to send the work to one of the Creative Cloud desktop apps and finish the work there or to save it as a .png to Photos.
Thank you for the answer, Sue.
This is indeed great to know that it's coming in a new version. Though I can not wait for it to be available and have no CC desktop application - only Illustrator CS6. The usual routine for me now is to save an illustration to Creative Cloud as PDF and download from there to finish the work in my CS6.
But seems like the current version of Draw can not help me… this is very bad news.
I realize that and I'm sorry there's no solution for this particular drawing.
Don't be sorry, Sue Draw is still a fantastic tool giving me a lot of flexibility with illustrations I'll just try to move my layers within the canvas border and be careful with it next time.
Have a wonderful day!