I‘ve checked from the web that Canon XA20 output either AVCHD or H.264/AVC MP4. And theoretically, FCP X supports both of these two format footages as it's said in Apple store. However, if you find it doesn't work and you don't want to bother so much about the further setting of FCP X, you can take my advice into consideration. That is, you can use some third-party software to convert your Canon XA20 footages to Apple ProRes 422 MOV, the best supported format for FCP. If you want to try, use video converter.
People usually use video converter to transfer the incompatible format files to their programs and devices, maybe in your situation, you can have a try as well. If you don't know how to use, here the guide for you.
wish you can solve your problem soon and good luck~