I actually meant like if I have an idea, plot and characters but when I want to start writing the "first book" then my mind goes plank. Then is it okay to write a 0.5 part before and from there on continue with the "real" series? Sorry if I was unclear before and I wouldn`t want to mess up the beautiful AtU series with my writing.
Ah! I see--go for it! Write whatever story, or part of the story, that inspires you to write :)