Sure, there are external options available, but part of the appeal of these svelte notebooks is just how thin and light they are relative to their direct competition. Who wants to haul around a bunch of adapters and accessories, Donglebook Pro worries aside?
According to The Verge, the new fingerprint reader is integrated into the keyboard on both the XPS 13 and the XPS 15. You can enable this option when you go through the configuration tool on Dell's website. The fingerprint reader is also compatible with Windows Hello.
And better still, you're not forced into one of the higher end configurations if all you want is a beautiful laptop with a fingerprint reader. The option is available on entry-level SKUs (starting at around $800 for the 13 and about $1000 on the 15) too.