What you ask for isn't QUITE possible. You always have to choose one of the Manuscript Papers that are offered in Quick Start. If you want yours, created as Robin suggests, to be top of the list, locate your user Manuscript Papers folder, create a sub-folder called 'a', put your template score in it re-named as 'aardvark.sib'. Well, it will be almost top of the list, there doesn't seem any way to knock the system's 'Common' category out of prime position.