Hi Muriel,
Unfortunately, there's not a great way to do this in AtoM right now without a workaround.
AtoM does not currently have an option to export all terms in a given taxonomy. Instead, you can export directly from a term view page, which will also include any descendant terms in the SKOS XML exported.
This means that at present, if you want to export all terms from a taxonomy, they will need to share a common single term as a parent. You could potentially move terms under a common placeholder parent as a workaround, but I understand this is not an ideal option.
I was also a bit surprised that I couldn't find a bug ticket or feature request for this in our issue tracker, so I've created the following Wishlist ticket so we can track this for future sponsorship and development opportunities:
Regards,