Is it possible to add a hardware clock to available_clocksource in order to select it as the current_clocksource found in: /sys/devices/system/clocksource/clocksource0
I don't think the item you listed has anything to do with
DS3231. This is simple to interface, and all the drivers are included. See https://raspberrypi.stackexchange.com/a/53395/8697
I may have misinterpreted your question. If you are proposing to use
RTC as a system clock, this is still not possible, as most RTC only generate low frequency clocks.