Can you put custom parts (HDD, GPU, Display) on each notebook?

We cannot modify the basic specs of any notebook. For instance, if a laptop only holds a single GPU - then we cannot modify it so that it holds dual GPUs. Also, we will not special order particular brands of hard drives, memory etc. We use the most stable brands available for our notebooks. Lastly, we do not allow for parts (HDD, CPUs etc.) to be sent in for us to install. There are no exceptions to this policy.