ASRock B450M Steel Legend Secondary M2 SSD

I've had much trouble with various NVMe to USB enclosures, so it might not be the best option, but you could turn it into an external USB drive.

This one has been pretty good to me:
https://plugable.com/products/usbc-nvme/

The only quirk I've experienced with it is that it actually matters which orientation the USB-C connector is connected. One way is slow and maybe even unreliable, but the other way is super fast and reliable. SHRUG

https://www.asrock.com/MB/AMD/B450M%20Steel%20Legend/#Specification

- 1 x M.2 Socket (M2_2), supports M Key type 2230/2242/2260/2280 M.2 SATA3 6.0 Gb/s module

That motherboard's 2nd M.2 socket will only support m.2 SATA. It does NOT support m.2 NVMe (PCIe) because it's not wired for PCIe devices.

If you have an unused and accessible PCIe slot, you can use something like this to add another NVMe drive:
https://www.amazon.com/dp/B01FU9JS94/?th=1

However, I see that the top PCIe slot is only wired for single PCIe lane and only Gen.2, so if you use a Gen.3 x4 NVMe drive on that slot, you're going to be limited to 1/8th of the maximum theoretical bandwidth that the drive can support.

The big slot at the bottom of the motherboard would be a good match for a card like this, if your GPU isn't so huge that it's blocking that slot.

If you don't even have a discrete GPU and using integrated GPU (G-series APU), then you can also use the main PCIe slot for this.