The STM32L431 EXTCLK MCU pin 14 and 15 need to be conected or not
The Reference Manual for your part states in section 6.2.3


  Hardware auto calibration with LSE (PLL-mode)
  When a 32.768 kHz external oscillator is present in the application, it is possible to configure
  the MSI in a PLL-mode by setting the MSIPLLEN bit in the Clock control register (RCC_CR).
  When configured in PLL-mode, the MSI automatically calibrates itself thanks to the LSE.
  This mode is available for all MSI frequency ranges. At 48 MHz, the MSI in PLL-mode can
  be used for the USB FS device, saving the need of an external high-speed crystal.


So yes, it does look possible.