I've got a M.2 (NGFF) WiFi card and I would like to use it on my PC, which, however, does not have such a port.
I know there are M.2 to USB adapters, but AFAIK they are designed to connect SSD cards, not WiFi cards.
Would it be possible to use one such adapter with said WiFi card?
There are different signals available in the M.2 slot and various keys in the slot to show which ones are (or, might be) available. So M.2 key B has two PCIe lanes and one SATA, key M has four PCIe lanes, key E has has one PCIe lane (all of them can have USB). Wifi cards use the latter and typically connect over PCIe (it's interesting to note WWAN cards used to use USB but some of the latest ones are PCIe).
Unlike PCIe cards, USB devices can't become DMA masters and as such building an enclosure capable of housing a PCIe device would be a challenge too big. It obviously is possible, it's just unusually complex and simply not worth it. (Note: surprisingly, NVMe adapters to USB now exist. That's still not generic PCIe though.)
You can use Thunderbolt enclosures, though.
I have the same question as OP, but for slightly different reasons: I'm looking for combo WiFi+BT, and ones available are usually USB nano, which are terrible range-and-obstacle wise. By BT dongle has half the range if plugged to the back of my PC tower, and then there's desk and other appliances in the way.
So having something that can do much better range but still be compact enough to plug into an USB port is very good idea.
However, the main obstacle is that M.2 wifi cards are usually in A- or E-key, while SSDs are now coming in M-key. Those aren't compatible, so highly doubt there will be an M.2-to-USB (preferably 3 or higher) adapters with A or E key slot...
So, the answer is: no, not really, unless you happen to have a really obscure M.2 adapter type.
This adapter "USBM2 Series Wireless USB M.2(NGFF) Card Adapter":
http://www.hwtools.net/Adapter/USBM2%20Series.html
claims to be doing what the OP asks (and more, since it is conceived for WWAN, and hence has its own SIM slot, and related logic, and power adapter...).
Not sure it's what the OP would like to use, though... it's definitely more complex and cumbersome than the typical SSD adapter. But as observed by 350D, the OP will need something to hold antennas at the very least.
EDIT: this answer is not good: the adapter being sold is still B keyed, which is OK for at least some LTE cards out there but not for a typical WiFi M.2 card. See the following picture, where you see an A+E keyed Intel 7265 WiFi card on top of a random B+M keyed SSD memory: the M keyed adapter on which they lie will happily host the memory (and at least mechanically, a LTE adapter like the one previously linked), but the WiFi card instead can't fit in.
It looks like this adapter could get you part of the way (exists in A-key and E-key varieties), you'd need an enclosure and find a way to connect a USB cable.
Or does it in fact do the opposite??
I believe the M.2 USB adapter will work. However, I agree with some of the comments that getting a M.2 to PCI-e adapter or native USB WiFi adapter would probably be the better solution.
