I haven't seen a USB adapter for M.2 NVMe SSDs but there are plenty of PCI-Express adapters you can mount the NVMe SSDs too . . . Do you have a spare PCI-E x1 slot in your machine?
itm,
pick up an M.2 PCI-E Adapter and install your NVMe SSD to that and then install the PCI-e card in a spare PCI-E slot in your machine. If you using Windows10 then it should be plug and play but if your using an different O/S like Win7 you will need to install a driver. If you search there are plenty of M.2 PCI-E Adapter for several quids.
Most of the M.2 PCI-E NVMe Adapters that I've found have a different physical connector to the one on my motherboard - the longer of the two lugs on the connector is closest to the backplate in my slot, whereas most of the adapters that I've found so far have the shorter lug closest to the backplate. The Sintech adapter in the link above looks like it has the right PCI-E connector for my board, but doesn't mention anywhere that it's for NVME drives.Yeah a M.2 PCI-E NVMe Adapter will fit in most PCI-E slots, the PCI-E standard allows any previous generation to fit into the current generation slot. I picked up my card at auction for cheap.
The connector on that isn't the same as the one on my motherboard - the longer of the two lugs on the connector is closest to the backplate in my slot, whereas on this one the shorter lug is closest to the backplate.Look at this adapter and pick up a cheaper version that looks the same.
For example this card has the correct type of adapter for my board:The connector on that isn't the same as the one on my motherboard - the longer of the two lugs on the connector is closest to the backplate in my slot, whereas on this one the shorter lug is closest to the backplate.
Well, according to the official ASUS specs that board has two PCI-e x16 slots, two PCI-e x1 slots and three regular PCI slots. I assume your graphics card is in the first PCI-e x16 slot so you should have a second PCI-e x16 slot below this . . . is there currently another PCI-e device plugged into your second PCI-e x16 slot or are you saying your Asus P8Z68-VLX doesn't have a second PCI-E x16 slot?