Hands are baked into the model. If you want C_arms on the cinematic mod weapons you'll have to go through and recompile each and every... that's what Inaki did to create the c_model weapons in the first place. The c_models have invisible hands, in which custom playermodel makers can create meshes that have the same skeletons as the HL2 hands, and they are bonemerged into place, simulating changing hands.
Basically something that's undoable by anyone but the most expert (and patient) modders. Its advanced stuff, no drag and dropping can fix it.
I know this is probably what you don't want ot hear but you're probably out of luck unless you know how to do it yourself.
If you want to keep the custom playermodel viewmodel arms, stick with vanilla.