I'd also like to know what is making you think the joey is being rejected? It's a very valid question because often times people think a joey is being rejected when it's really not so they end up hand raising the joey when it wasn't necessary.
Breeders normally feed their producing pair an extra half or full portion of staple while the joey is in pouch and nursing joeys. This not only increases the protein but also all of the other nutrients in the staple keeping the overall diet still balanced yet offering mom more of what she needs to support a good milk supply and her own body what it needs.
This also provides a portion to the joey when Mom starts to ween the joey and introduces normal food. As mom stops needing the extra portion the joey starts to eat it on their own, as early as 4 weeks oop. So it also gets you in the habit of providing the joey a portion.
As far as hand feeding a joey IF it is indeed rejected....
Keep the joey warm, if it's cold it will not eat.
Hold the joey vertically when feeding to prevent aspirating.
Start off with one drop at a time, one drop doesn't seem like much but to a little joey it's a lot, once the joey swallows the first drop give another and continue until the joey is licking it up on it's own, then you can give a steady flow of food from the syringe instead of drops.
Somewhere on the site Tiakristen provided there is a chart for how much to feed and how often to feed joey at each stage I think. It's been a while so I can't remember where exactly it is.
Now if you take over the joey completely from the parents, you will also need to stimulate the joey to pee and poo as the parents would if it was in their care. The parents stimulate the joey by licking the cloaca, you don't have to lick the cloaca
use a wet qtip and gently rub it on the cloaca to stimulate the joey to pee and poo.
Supplement feeding... I know some breeders have done it and have been successful to supplement feed a joey and still let Mom nurse it when its not getting enough from mom alone. Personally I feel when you supplement feed this reduces the milk supply from mom even more, however the benefit is mom and dad are still the primary care giver tending to the joey instead of the human so I can understand why it's done. That's going to be one of those things you have to make the decision on yourself if you feel it's absolutely necessary or not.
Hope this helps some and good luck!