To answer your question in a word: Yes.
It is entirely possible to engineer a specifically defensive robot, an entirely offensive robot, or a versatile combination robot. Nowhere does this year's FRC manual ban defensive play, and the allowed motors and materials provide a team with more than enough possibilities to build a robot that can adapt and play offense or defense accordingly. This type of versatility is looked upon favorably by other teams and is encouraged by FRC.
Many veteran teams put valuable time and resources into helping rookie teams through guides and manuals to spur your creativity or help with designing systems. The first that comes to my mind are the resources that team 1114 offers on
http://www.simbotics.org. These include the Simphone app and the "kitbot on steroids" to assist teams with strategy, planning, design, and execution of the robot. I would highly recommend using these and also utilizing the amazing database of useful information that is Chief Delphi right at your fingertips. These can answer your future questions and give you tips and ideas for your own robots.
Good luck this year, welcome to FIRST.