For cup and cone hubs it's fairly easy. The axels are usually a standard size and thread. Just get a longer axle if required and some spacers.

For cartridge hubs is pretty much on a hub by hub basis. If there is enough axel you can just use spacers but sometimes they have to go between the lock nut and the frame. Really you want the part of the hub that contacts the frame to be serrated for grip and you may get stuck with smooth spacers.
If the axel isn't long enough then there is often little you can do. The axles typically have a smooth center section with sholders that the bearings press against and threaded ends. Different hubs have different size bearings and bearings at different positions in the hub shell so axles are not interchangable