Putting anything inside your laptop will already void the warranty, unless you plan to put it in an empty space that is accessible from the outside, like an HDD slot.
While that is commonly believed, it is mostly a myth. I mean, obviously, different rules apply in different jurisdictions, but generally, if it is
your laptop, it is
your laptop, and as such, it is exclusively your decision what you do with it and the seller can not put arbitrary restrictions on that, and that includes that they can not disclaim liability for defects in a product just because you used the product in a way that they don't approve of.
Obviously, something different can apply for voluntary warranties (i.e., in Germany, Garantie as opposed to Gewährleistung/Sachmangelhaftung).