What precautions, if any, should I take to use external clock source instead of crystal/resonator?
You can apply a 32768 Hz oscillator signal to the XTAL1 pin - see the input specifications on page 5 of the datasheet. The PLL should be able to lock onto that and generate the 4.19MHz clock. The jitter of the clock source is not important, since any jitter will be averaged out by the decimation performed in the sigma delta and post filtering. However, the absolute accuracy and temperature drift of the clock source is important since the output update rate and placement of filter notches is a direct function of the clock frequency.