For me, I have created many different solid color backgrounds in various standard sizes... I just used Microsoft Paint and saved each as a .BMP file. I then can insert these into a project where ever I want, I mainly use these as backdrops for text so I get the exact color I want. 

You could also move all the material in the timeline mode portion of a project to the right, opening a gap between time 00:00:00 and the first visual frame. I do not like this method as the program will try to compensate for the gap as if it was an error. 

I hope that helps