Unable to add custom fields in AR Invoice (Header & Line) – Receivables objects not available in App

We have a client requirement to add custom fields on the AR Invoice page (both header and application line level) without using Descriptive Flexfields (DFF).

Issues faced:

• Receivables objects are not available in Application Composer, so we cannot create custom fields there.
• Unable to find any official documentation or steps for adding custom fields on AR Invoice pages without DFF.

Questions:

• Is there any supported approach to add custom fields on AR Invoice header and line level without using DFF?
• If Application Composer cannot be used for Financials objects, what alternative options exist?

Any Oracle documentation or real-world guidance would be highly appreciated.