at first you take the atomic masses from the periodic system of elements. We take 1mol from every component - that is the atomic weight in g:
1 mol Mg: 24,3 g
1 mol Sb: 121,8 g
1 mol Na: 23 g
To fabricate 1 mol Mg3Sb2 we need:
3*24,3 g Mg + 2*121,8 g Sb = 72,9 g Mg + 243,6 g Sb.
The mass ratio can be changed of course by a constant factor, if you need more or less as 316,5 g Mg3Sb2.
Now the Na doping. Therefore we need the expected concentration of Na. Lets assume, you want to have a concentration of 1017 atoms Na per mol. A mol contains 6*1023 particles (molecules Mg3Sb2). That means you need
23 g Na*1017 /6*1023 = 3,8 µg Na. You can calculate the volume concentration if you have the material densities.
Obviously the only issue here would be trying to add 3.8 µg Na to anything. One could perhaps make up a more concentrated solution and dilute accordingly. Otherwise doping levels would have to be increased at least a thousand-fold (10000 X would be better) to limit errors in weighing.