Is it possible to use an M.2 to USB adapter designed for SSD with a 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.