How can I add a Custom Field (or Customer_name) to a picklist?

How can I add a Custom Field (or Customer_name) to a picklist?

Our workflow is Shopify-->SO -->Picklist-->Package-->Shipment.
We would like to have the Picklist template to include a custom field "Customer_Notes" (we have created this field already). This way we get alerted when a customer has notes on their account. For example "do not ship via UPS" or "customer always picks up, do not ship" etc. 
Is there a way?