If you open your AXF file in Studio, double click the Feature Layer whose related table you want to edit (it opens a new window with that Feature Layer's forms and scirpts, etc), then double click the <FORMS> icon (opens a window with your Feature Layer's forms in it, EDITFORM, IDENTIFYFORM, etc), then click on the EDITFORM, the button for "Related Tables..." will become available. From there you can edit the related table's forms how you like.
I'm honestly not sure how this behaves in ArcPad (how the related table's forms are fired with respect to the edit form), I only recently noticed it, but hopefully it helps you get started.
Good luck!