If you already have one M.2 SSD in addition to the hard drive, you cannot add another.  You can replace the 2.5" hard drive with a much faster 2.5" SATA SSD, though.  If you have only the hard drive, you can install an M.2 NVMe drive, which is the fastest option. There are still some 7200 rpm SATA hard drives, but they're nowhere near as fast as a 2.5" SATA SSD.  
Hello all, My sons G3 3579 hard drive drive failed today.  The system shipped with the 1Tb 5400rpm drive.  Can I just remove the drive, and insert a M.2PCIe 512Gb drive, and re-install the image?  Is there a 2.5" drive option faster than 5400rpm.  Any other recommendations?  Thank you,Chris
G3 3579 - 1Tb drive failed, warranty ended in January