I've seen some people do it, but I need some explanation.

Say I want to use a gun in payday that is in, eh, let's just say from Battlefield 4. Now, if I want to put a similar gun model and such into usage with Payday, do I need to create the model and textures and etc. from scratch? Or is there an easier way to port. I was just kinda thinking about retexturing and copying models and what not.