In a form with a subtable, is it possible to add data to the subtable without opening the subtable's form?

Have you tried clicking on the subtable's name on the left? You can add records to the table directly and relate each record back to the parent table. Maybe I am not understanding your question.

what I need is to fill the subtable in the same 'parent' table form. When I click on any subtable row, it opens the subtable form

Sure.. you can put a script behind a button on the parent table.