Imagine I have a car bought with no extras. And imagine my friend has the same model but with all extras (e.g. a steering wheel with built-in controls, or a electric seat, etc.). Now imagine his car broke but the said extras are still working. Under what conditions is it possible to upgrade my car? I notice that the fuse box for example has all slots for the extras empty. If I install a fuse and connect the component wires in the same way I find it on the other car will it work? Have you ever heard of someone doing this?
Retrofitting options on most vehicles will require you to check for both the fuse and whether the vehicle is "prewired" for the option. The ease with which you can wire in options on a vehicle not prewired for options differs from manufacturer to manufacturer. What vehicle are you considering modifying?
It completely depends on what model and year of vehicle.
In most modern cars each module such as the BCM, Instrument Cluster, Radio, Indicator stalk etc etc, is preprogrammed. In some cases this can be changed through diagnostic equipment but in some cases such as Landrover, Vauxhall and others it cannot. You would need to find an experienced auto electrician to remove the memory from the effected ECUs/Modules and manually change the contents of the memory.
Wiring should be relatively straight forward to an experienced technician. It will probably involve inserting some pins into electrical multi-plugs. But if you have the donor vehicle you can just follow that.
Although it could be a lot easier or a lot harder than this as you haven't specified any info about your vehicle.