How to convert nature of a BPM stage for New Item Request or Change Orders?

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Hello,

In Procurement BPM workflows there is an option to convert a BPM stage from serial to parallel, or vice-versa. And system also allows option to add a new stage. For BPM task for New Item Request, however, I'm unable to find this option of editing nature of the stage.

We have a requirement wherein all NIR will go to a common group for approval, and post that system will segregate a NIR based on the item class and further send it to different approval groups. 

Looking forward to hearing from anyone who is able to suggest solution for this.