This is possible in Order Management. You are likely just running into item attribute issues. Are you using a Copy Template and Name as Service Template when creating this item. Also, service items are usually Generic, Bill Only line flows.
Please review - Service Item Setup and Pricelist Configuration FAQ (Doc ID 167618.1)
Are Promotional Modifiers for Service Items Supported? (Doc ID 1116158.1)
How To Set Up a Promotional Modifier for a Buy One Item Get One Item Free Promotion (Doc ID 226379.1)
How to Price Service Based on the Product being Covered Using Order Management (Doc ID 1640138.1)
"Also, service items are usually Generic, Bill Only line flows."
This is my scenario:
Promotional Item = Service Item
Get Items = Inventory Items
I am using Generic, Bill Only with Inventory Interface because upon Book Order, I only need my Service and Get Items to be in AR Interface tables directly. I should not be doing Launch Pick Release, Ship confirm, etc.
Is Generic, Bill Only with Inventory Interface possible with my scenario?