I'm not an expert in this. But my feeling is the following:
There are no out-of-the-box integration scenarios for this.
But you should be able to use the a2x-interfaces in ByDesign for purchase orders and PO acknowledgments to cover Scheduling Agreeements, Order acknowledgemen and Order changes, and there's also one for invoices.
I am not sure if there are interfaces for stock information. In that case I believe it would be easier to use the SDK to build specific business objects for this and integrate that into the stock information in ByDesign.
Best, Christian