Order Management sales order Header Section - using Page Composer how do I add a custom field?

Summary:

Create two new read only sales order header fields to hold values that are populated in two header EFFs.

Content (please ensure you mask any confidential information):

I'm attempting to add two fields to the Order Management sales order header area to hold values that are contained in two of the header EFFs without success.

Firstly, I cannot seem to add fields correctly to the sales order header and secondly if I can achieve this how would I populate these fields following the population of the two EFF at sales order creation? An OM extension? An OM pretransformation Rule?