Huge thanks for your help. But that won't make it.
Maybe i could work something around if you could answer this question for me :

Can you "unlink" Heading 6 from the Headings 1 to 9 hierarchy?
Is there a way that i can customize this heading so it loses everything that links him with its brothers?

I will tell you why.

1) I need an independent headings so i decide on his priority level on the bookmarks tab. (I can do this with a heading different from heading 1 to 9)
2) I need to use a heading between 1 and 9 for my appendix heading because i need to use it as chapter so my appendices has the chapter number into the page number. (I can do this already, but dropdown menu only lets me pick from heading 1 to 6)

Problem is... there is a contradiction said in those 2 things above. Both can't be possible at the same time.

So i would need to figure a way out to take one the heading (lets say heading 6) away from that pre-built hierarchy and unlink it.

So i can use the heading from the dropdown menu for my chapter number and choose its priority level on the navigation pane in the bookmarks tab.