Paragraphs which dispaly in the navigator in the Headings part have an outline level defined in the Outline & Numbering tab of style definition.
They are then displayed in occurrence order. Their hierarchy reflects the outline level from paragraph style definition.
Accordingly, your "section" heading must have the lowest outline number of all, i.e. 1 which is already taken by Heading 1.
If you don't want to restyle all your existing headings, you could think of modifying outline levels for Heading x and using Heading 10 as your new section paragraph style. Unfortunately, built-in Heading x styles are protected against level modification (I guess that they are internally tightly linked to the outline/contents engine).
The correct way of doing the level reallocation is to use Navigator powerful features. The second toolbar line contains 2 icons (those at extreme right) to promote/demote (up/down one level) the selected chapter.
Then, with Headings displaying only level 1, proceeding from end to beginning (in case you didn't shrink the tree view), click on a chapter heading and click on the "demote chapter" icon.
This will change all Heading n to Heading n+1, freeing Heading 1 for whatever purpose you wish.
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