The 32.768 kHz crystal oscillator (LFXO) is designed to work with external sources.
The device can use a rail-to-rail clock, where the signal should be applied to the XL1 pin with the XL2 pin left unconnected. To enable rail-to-rail clock, set XOSC32KI.BYPASS=Enabled.
Using an external source requires that CLOCK.LFCLK.SRC=LFXO.