ratiometric ADC, is it feasible to separate reference voltage and excitation voltage by an amplifier
It certainly looks feasible, but first make sure you NEED to do it.  The 24 bit ADC gives you an awful lot of dynamic range that you don't need, and plenty of bits to spare.  You often use them to sample small signals without worrying about preamplification.

So, work out your noise budget, figure out how many bits you need, and do what you need to get them.  Try to remember that an engineer who consistently exceeds specs instead of meeting them is wasting time and money.