You can't really. It's common to just put your regiment and divison tags (without rank, e.g. 20[9th]) in front of your Nickname as a Clan tag. You can do so in the BF2 login screen.
Alternatively you can create a new account with a random email adress(blablabla89834@bla.net) and use your full name as it appears in the company roster (20[9th]Pvt.Tankbuster).