Can I subscribe an additional plan when I already have an existing one? How is the service time calculated?

If your device is already bundled with a Blurams Guard that is not automatically renewed, you can continue to subscribe to new ones. The new Blurams Guard will automatically take effect after the current one expires;

if your device is already bundled with a Blurams Guard that automatically renews, you need to unbundle the current Blurams Guard or cancel the automatic renewals before you subscribe to a new one.