How to add a filter/parameter to an Extended EndPoint

Hi, 

I have extended Default/18.200.001 endpoint to include SubContracts like this http://localhost/test/entity/Subcontracts/18.200.001/Subcontracts/

I can retrieve all Subcontracts, but I would like to filter to get 1 SubContract back as it will hit the max size limit eventually.

Is there a way to do this like its possible to do with Vendors http://localhost/[sitename]/entity/Default/17.200.001/Vendor/AAVENDOR?$expand=MainContact ?

Additional notes:

This is what I have currently and it works except it returns all subcontracts

http://localhost/test/entity/Subcontracts/18.200.001/Subcontracts/

I would like to have it working like vendors to filter to a subcontract like so

http://localhost/test/entity/Subcontracts/18.200.001/Subcontracts/SC00001 to retrieve only subcontract SC00001

and 

http://localhost/test/entity/Subcontracts/18.200.001/Subcontracts/SC00001?$expand=DocDetails to retrieve only subcontract SC00001 and expand DocDetails.

Thanks in Advance

Mark