Very doable if you keep it clean.
Take all the balls out and mic them for size. Keep a count on them too. They usually come in standard sizes, and over/under sizes. Put aside all the old ones and buy all new ones from most any bearing supplier. They are not usually stainless, but just relatively hard steel, and about $3-$25 for
100. Most screws we had done take about 100-200 balls.
Clean the screw well. Pull any seals and soak the nut in kerosene overnight, then alcohol for a few hours. Shake and stir occasionally to get the little bits and old oil/grease out.
For assembly use grease to hold the balls in place as you wind the nut and fill return tubes, winding the screw a bit as you go. If you are using an oil-lubed nut/screw, then use any light grease that cuts well with Vactra 2 oil, as you want it to wash out later. If it is a grease packed nut/screw use red axle or whatever other grease you are told to lube the nut and linear ways of the machine with. It is a relatively low speed bearing so fancy grease is not usually necessary.
The big trick here is when you start winding it on, to make sure you understand how to wind it on and off with most of the balls in place, and realize that there are two separate ball tracks opposing each other for preload. Use the grease to hold them in their tracks as you wind it on. It's easy to tell: One way works, and all the balls stay in the tracks and feed into the return tubes as you wind. The other way, and they all fall out on the floor. Pack the return tubes as you finish.
In reality you'll do it a few times (on and off, "dang-it", on and off, "dang-it again", and on...) then you'll understand how it works.
We have reloaded many screws for various reasons, including the one you mentioned, where some bone-head unwound it off the end of the screw, and then tried to put it back on with not enough balls.
If the screw and nut bearing surfaces are in good shape, they usually turn out beautifully smooth again. (However, if the tracks are rough and pitted, just throw it away.)