Yes, you can use the Boundary system to secure your garage or shed using either Motion or Contact Sensors.
You should note however that you would not be able to apply for professional monitoring with police response of these types of properties as this is not covered under the British & European standards for monitored intruder alarm systems.
It is also possible you will see range issues when there is a large amount of open space between the hub and the sensor or there are thick exterior walls in the way. We don't recommend having sensors outside your home but it is possible with a few minor caveats!