Is it possible to FTP files out of a M340 PLC with SD card and data storage via the USB port?

Yes. The M340's USB port doesn't just handle programming traffic.  Windows treats it as a virtual Ethernet adapter (the "Usb PLC" service), and the CPU answers on it at the fixed IP address 90.0.0.1. That means you can point a standard FTP client at the PLC over USB .

This applies to the BMXP342000/341000/342010/342020/342030 family, and the memory card needs to be the file-storage variant (BMX RMS 008MPF / 128MPF)  since that's what actually exposes the file storage over FTP.