I seriously doubt your warranty won’t let you install RAM. If it actually does, install it anyway, then just take it back out before sending it in to Dell for repair if and when it breaks. The RAM is not really going to harm the computer, so they’d be out of line voiding your warranty for it.
There is no problem with adding additional RAM and it won’t void any warranties. These computers are designed for upgrades or they would have sealed the insides.