adding elements to a 3dc model is impossible (idk bout the 3doc but i'm quite sure it ain't working there either). the only thing u can do to change your 3dc model is hiding faces away from the model with shStudio.
one thing that might work is open 2 3dc files and save em as one file merging em toghether. dunno the result u'll get to but i'm 100% sure your dds file won't match your 3d model