When I needed to do the same thing (both m.2 motherboard slots filled), I just ordered one of these NVME to PCI-E adapter cards with the new SSD.
NVMe PCIe Adapter with Heat Sink, M.2 NVMe SSD to PCI: Amazon.co.uk: Electronics
Just plugs straight into a PCI-e x4 (up to x16)_ slot et voila - extra NVME ssd.