It is not difficult.
From Brekeke SIP Server, you can use Brekeke PBX as a SIP client.
From Brekeke PBX, you can use Brekeke SIP Server as a ITSP on ARS.
Settings are depending on your environment and requirements..
The easiest way is the use of Upper Register at the PBX side to register to the outside Brekeke SIP Server.
(Search "Upper Register" in the document for more details.)