Look on this site for joining the iOS beta program. There is an app called Virtual Station only available via invitation in the beta: you install Apple's TestFlight app, and in it you download Virtual Station on a spare iOS device. Note, from experience: iOS 11 is very unstable with the Virtual Station, so I recommend a device that is not running iOS 11: I got an old iPod Touch 5th Generation and used it.
Note, that is half the battle to getting SmartWatch success: you then need a Raspberry Pi or something to run homebridge, which allows you to run QSesame (see above in this thread) — and that allows you to use Siri and Apple's Home app to control the lock. That (Apple's Home app + Siri), by the way works on an Apple Watch, and thus you can use your watch to enter and leave the house.
So if you have all that working, I would also recommend using a Mac + Apple Configurator (on the Mac App Store) to put the spare iOS device in Single App Mode and lock down the screen etc: this way the device will only run Virtual Station, prevent you from switching to another app, and if it crashes it will restart it.
I have that exact set up, and the Virtual Station stays up pretty much permanently (and the Raspberry Pi is similarly stable) and thus I can reliably use my watch to unlock and lock the house.
But, as you can see: not for the faint of heart!