Answer
In Enterprise Explorer, on both Apps9 and Apps10 (EE's architecture is unchanged between the two), you cannot nest a table-style Custom Logical Unit/Table window inside a Custom Page or Form. You can add new tabs to existing STANDARD ('core') forms, but only where other tabs already exist there as standard — you can't create an arbitrary new form-plus-nested-table combination purely through the Custom Page mechanism in EE. The Aurena client's fundamentally different architecture is considerably more flexible for this kind of header/detail custom-page layout, so this specific requirement is realistically only cleanly achievable once genuinely working in Aurena rather than continuing to build it in EE.