No. James Cameron's Avatar: The Game (Wii/PS3/PC/X360/DS) forces a single narrative choice at ~30 minutes: join the RDA or the Na'vi. You cannot play as an RDA soldier freely—you are the RDA signal specialist Able Ryder if you choose that path.
Choose RDA at the branching point—that's your only way to "be" them. No sandbox RDA mode exists.