It is possible to use external capacitors after disabling the internal capacitor.
When using external capacitors, the load capacitance (CL) is the total capacitance seen by the crystal across its terminals. It is calculated by the following equation.
C1 and C2 are the external capacitors. Cpcb1 and Cpcb2 are stray capacitance on the PCB. Cpin is the pin input capacitance on pins XC1 and XC2.
When using external capacitors, disable the internal capacitor by setting OSCILLATORS.XOSC32M.CONFIG.INTCAP to 0.