Is there a way to use a predefined template to create single player level?
As far as i can see, it's required to create everything from scratch every time.

that is correct! there is no current way to create/save a sp template. What one could do is create a draft of an existing sp level, but it doesn't really solve the issue, as it would replace your older draft, and that is mainly the reason why I haven't yet finished the hard version of my first published level.

I asked about this a while ago and Fizzer said something on the lines of new players would straight away make levels and wouldn't have any saved single player templates.