Hey Asad,
can you try to extend from the root node?
There are a some BO you can't extend. I think this is one of them.
What you can try to do is to find the parent node of the BO and extend it from there. Sorry I don't know what is in the case of LabourResource.
For comparison: I faced similar problem when trying to extend AppointmentActivity. But it works in Activity because AppointmentActivity is a type of Activity.
Good luck
Cheers
Linh