I succeeded to plug a 7th GPU on my motherboard with the "NGFF M.2 To USB 3.0 Transfer PCI-E Riser Card Adapter".
Thank you guys for answering my question and giving your personal experience. So in return here's how I did.
First I contacted the manufacturer of the motherboard to ask if it's possible to use the M.2 Key slot to connect a GPU. It's not possible with all the motherboard apparently. Here's my motherboard:
PRIME Z390-A ASUSTeK
Then I bought this M.2 Card Adapter to connect into the M.2 socket of the motherboard:
https://www.ebay.ca/itm/122990821026
So as usual, you plug the GPU on a riser and the USB cable you remove the PCIE X1 and instead plug it on the M.2 Card Adapter". See photo:
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It's important to power properly the GPU so like always, I use the VGA cable of my of PSU to plug into the GPU and the riser with this 8 Pin PCI-E to 2 PCI-E 8 Pin (6 Pin + 2 Pin) Power Cable. I always buy that cable from that seller on Amazon and never had issues:
https://www.amazon.ca/gp/product/B08KP9R92V/ref=ppx_yo_dt_b_asin_title_o06_s00?ie=UTF8&psc=1
Then in the BIOS of my motherboard, in order to configure M.2 slot in PCIe mode, I go to BIOS > Advanced > Onboard Devices Configuration > M.2 PCIe mode and changed the dropdown to ‘Enabled’.
Et voilà !
I noticed I could add another M.2 Card Adapter on my motherboard (same process) to have 8 GPUs but for now I will monitor closely how it goes with my 7th GPU. Plus, not all my mining rig frames are made for 8 GPUs which is now an issue.
Hope it can help other miners.