Flexfields attributes not available on Goal Plan Template

Summary:

When we try to use HDL or HSDL to create Goal plan Templates, flexfields do not appear on the attributes list

Content (please ensure you mask any confidential information):

From Data Exchange -> View Business Objects -> by selecting the business object (In this case Goal Plan), normally there should be a tab named "Flexfield Attributes", we checked on other business object details and it's there. We would like to create an HSDL Template to create Goal Plans, but we are not able to fill custom attributes, while the standard attributes are all available.

How can we do to add the custom attributes on the HDL/HSDL template?