How do I add sensors (such as leak detection) without having to sign up for a paid monitoring service? Can I install a “base station” and use it without the monitoring service?
TIA
Welcome to the Wyze User Community Forum @ic897d! 
Glad to have you aboard! 
Yes. You can install the Wyze Sense Hub without subscribing to the HMS monitoring. However there is NO Self Monitored Alarm.
Without the HMS Subscription, there is no way to Arm or Disarm the alarm and the keypad is useless. Most of the hub settings are also inaccessible like the audible chimes for the sensors.
There are many users who use the Hub for home automation with their sensors without the HMS Subscription. After you install the Hub, just don’t activate the monitoring. You can then install all your sensors within the app and the hub will be able to communicate with them. You can use them for rules and routines or other home automation tasks.
If you need more info, @WildBill is one of the many users who has a hub operating in this exact capacity.