You may use a Nickle RTD in the MAX31865 in place of a Platinum RTD. Generally, we recommend using a Platinum RTD because it has the highest accuracy, but a Nickle RTD can work as well.
It is important to note that you will not be able to use the same equation provided on page 10 of the datasheet to calculate the temperature. Generally, most temperature sensors will provide a lookup table in their datasheets that you can use to extrapolate the temperature from the measured resistance. It is also important to note that Nickle RTDs have a temperature range of -100°C to +300°C, whereas Platinum RTDs have a range of -260°C to +650°C. As long as you plan to stay within the appropriate temperature range, a Nickle RTD should work fine.