You are not able to find Custom BO and it's Extension fileds created via PDI in Data Workbench.
It is not possible to extend the SDK created fields via Standard ODATA service. Same can be enabled via Custom ODATA service. This is standard system behavior.
Create a custom ODATA service and you may extend both KUT and SDK created extension fields in it.
• Go to Admisnitartor workcenter.
• Select ODATA Service Explorer
• Select the button Show and Custom Odata Services from dropdown.
• Select NEW to create new custom ODATA service.
• Select Data Workbench enabled (optional)
• Edit the Custom ODATA service
• Select the required Custom BO under Select Business Object
• Expand and select the erequired Entities
• Now all the Extension fields related to the selected entities will be available in the right hand side window of ODATA Service
• Select all the required KUT & SDK extension fields and save your custom ODATA service.
Now the selected custom BO along with it's Extension fields will be available in Data Workbench.
KBA , SRD-CC , Cross Components , How To