Could a group of survivors resupply metallic cartridges with hand-operated tools to be used on small arms in their shelter? What would those tools be and how usable are these handmade cartridges? Alternative methods are acceptable too.
Assuming the guns they have are more or less modern types made to use modern metallic cartridges, the answer is a firm "maybe".
First, there's the issue of obtaining suitable metal. Cartridges have been made from cartridge brass (70% zinc, 30% copper) for more than a century, and no really equivalent substitute has been found (though mild steel and aluminum have been used at various times, especially for cartridges not intended to be reused). Of course, one source of cartridge brass would be melting down cartridges that are beyond reloading, but otherwise, in a post-apocalyptic world zinc will be the big problem.
Copper is everywhere in the modern world -- house wiring and expensive roofs, for instance. Zinc is around, too, but it's harder to get into a usable form, since the most common appearance is as plating on galvanized steel. Cheap flashlight batteries may turn out to be the best source, though the older they get, the less usable zinc they'll contain.
Fortunately, for single shot and double barrel rifles and shotguns, and revolvers, the cartridges don't have to be all brass. Before there were modern brass cartridges (about the same time percussion caps appeared, in fact) there were cartridges that used a short brass base, which held the fulminate primer, and a paper powder capsule that also held the bullet. Important to note that these could be made compatible with existing firearms -- your granddad's .38 Police Special revolver could be fired with one of these 1820s technology cartridges, with no alteration to the gun.
So, there are several levels of "resupply" here -- we'd start with storing large quantities of commercially made ammunition, which can be depended on to last twenty years or so in good storage conditions. Then we'd reload -- primers and reloading powder can last longer than loaded ammunition, and the lead or jacketed bullets have a shelf life of centuries (at worst, exposed lead will get a coating of white oxide that will wipe away).
The early 19th century cartridges used a fulminate primer vaguely similar to modern Berdan primers, but most modern ammunition in the New World uses Boxer primers (the difference is that Berdan has the "anvil" as part of the cartridge case, where Boxer has it as a separate piece that's inside the primer). Contrary to popular belief, primers can be recycled, up to a point.
Mercury fulminate is relatively easy (if hazardous) to make, and was the first primer compound -- however, the mercury destroys cartridge brass if it's not cleaned very promptly after firing. The next easiest primer mix is that referred to as "corrosive" primers -- a mixture of a chlorate oxidizer, sulfur, and often phosphorus sulfide and metal powder to improve ignition of the propellant. Chlorates can be made from table salt or from chlorine bleach if electricity is available, sulfur can be stored indefinitely, and the other parts are optional improvements.
Reconditioning and refilling the primer itself, for either Berdan or Boxer type, is relatively trivial once the compound is produced. Like the cartridge case, primers will have a limited life, but they undergo less reworking and the brass is thicker, so they ought to last at least as well as rifle cases, if possibly not as long as pistol type cases.
Once the brass is too old to reuse (for straight-walled cartridges, this could be fifty or more reloads per casing, though for bottleneck rifle cartridges it's closer to a dozen times without special techniques), the base can be cut off (because the failures are always at the mouth or shoulder) and used with paper powder capsules. Given enough powder and primers stored, your survivors could keep shooting for a couple generations. In the end, if there are still primers and propellant when there's no more usable brass, they'd get down to recycling the off-cuts to make more case bases. It's actually pretty unlikely they'd be able to keep their guns working this long, so that's unlikely to really come up.
Are you asking if they could make and assemble all the parts of a complete cartridge?
Or are you asking if they could pick up empty casings, fill them with propellant, put homemade bullets in, put primer on the back, and so reassemble a cartridge out of the reused casing and otherwise new components?
If you are asking the second question, the answer should be yes, if they study what is needed and gather the necessary supplies before the disaster that they are survivors of.
I have read that in the 19th century the Sioux Indians, with basically stone age technology, invented a way to reload use metallic cartridge casings with loose gunpowder which was cheaper to buy than compete cartridges, and with homemade lead bullets which were easy to make at home with a lead supply, bullet molds, etc.
I also recently saw an episode of Death Valley Days, "A Bullet for the Captain", 3 January 1959, based on that Sioux discovery. In the episode someone was about to be executed for selling cartridges to the Sioux when it was found that the Sioux had invented a method to reload casings to make complete cartridges. The Death Valley Days episodes were all based on historic events but often very highly fictionalized.
So if your group of survivors includes any survivalists who spend most of their time preparing for the apocalypse those survivalists will probably have every thing they need to reload cartridge casings. And if not, maybe some of those educated 21st century people will be as inventive as stone age Sioux Indians and invent a way to reload cartridge casings with the available materials and tools.