After a rainbow/hash table is salted, is it possible to resalt it after finding the correct salt instead of generating it all over again? If not, what is preventing you from doing so? How can you find the correct salt for something?
No. Every hash would have to be recalculated, essentially reproducing the work that was needed to create the original rainbow table.
Hashing is basically a mathematical formula. You have a formula (hashing algorithm) of X + 5 = Y, where X is the password and Y is the hash. You use a rainbow table to store all possible values of X with their Y results. If you change or add a salt then the formula is now different too (like X + 6 = Y). So while the values of X wouldn't have changed the values for Y would all be different and need to be recalculated.
Typically the salt is stored alongside the password hash in a database and is not intended to be a secret (at least to those with password database access). If it isn't stored there it is either stored elsewhere or is a known piece of information (username, system secret, SSID). The source code of the software that validates logins will typically reveal how the salt is obtained.