You can find the templates here: Preferences > Flash Builder > File Templates
Product Manager, Flash Builder
I have to say your answer is obviuos. I belive that all or most of people using Flex/Flash builder do know well where preference are located. In this case we can not do much with that templates, only editing the existing ones. My question is different:IS IT POSSIBLE TO ADD (AND EDIT) CUSTOM ONES?. I guess now!
I can confirm that we do not provide the option to add new custom templates - you can only edit the existing templates.