Hi Ravi - Chapter 8 "Customizing Workflow" in the buyer_customization.pdf covers this topic quite well. Typical customization is to add an Action trigger during a certain order state. For instance, run custom logic when "Ordering" or "Cancelled", etc. You do this by creating a Workflow Extension file under the variant workflows directory.
Just note that you can only customize workflow post Requisition Approval. If you try to hook into the code before that, your customization is rejected on start-up. For pre-approvals, there are other ways to add logic, typically through a submit/approve hook.