How to extend Custom BO's field to a standard BO

Hi,

I am planning to create a custom BO for a custom maintenance table and extend the fields that I created to invoice request/customer invoice xbo's. But unfortunately I cannot find any node upon the creation of extension scenario. Any idea on how can I extend these fields to an existing xbo? Or any other approach that I can do? I have attached a sample screenshot for reference. Appreciate all the help.

Thanks and Regards,

JM Angeles