As a new unique weapon to register a branch improvement? (S.T.A.L.K.E.R.: Call of Pripyat)

Just say, I'm not moddel. But sometimes doing something for myself. This time I wanted to change primary weapons Degtyarev. Have also created unique Kalashnikov, who eats bullets larger caliber and therefore deals more damage. How to make so that it can be improved? On the Internet anything about the editing of the branches of the improvements I could not find it :(