Can you use existing window and door sensors from another company or do you have to buy new ones
The Ring Retrofit kit allows you to use existing wired contact sensors (door/window). You cannot use any other type of wireless sensors, nor other type of wired sensors at this time (like motion or glass break).
Reg. price for the Retrofit Kit I believe is $199. Since each Ring contact costs approx $20, your break even point is 10 contact sensors.
I believe there is a limit of 8 zones as well.