This is one of those "the answer seems so obvious, I wonder if I'm missing something" questions.
After typing "Reorganize storage cabinets over workbench" and deciding I was done with that action group, telling OmniFocus to create an aunt would put the next action where the cursor is. "Clean out garage" is the parent, "Park", "Sweep" and "Reorganize" are the children, and the action about to be entered is a sibling of the parent, an aunt. Does that clear it up?