Can't you just obtain a backup of the original?
Anyway 3 main types of undub
1) It is already in the game so someone makes a code to jump to it/force it.
2) It is the same audio format and content as the other region so the other region's ROM is opened up and things copied across.
3) It is a different audio format/content/setup from another region (typically Japan will voice more than most people dubbing things in English care to pay for so will have less but there are examples of more being added for Western releases) but related enough that you can take the time to convert between formats or adjust setups to work as things should.
1) Might be easy (I have done things in one line cheats
before) but also might be somewhat more involved.
2) Copied across tends to mean overwritten so you will be grabbing a given region's anyway.
3) Also tends to involve overwriting.
Barring bandwidth issues The only reason to really do this sort of thing is because the undub also contains other hacks you want and will lose by getting the "original".
Edit. 3a or maybe 4) might be when the undub is difficult because of lost code between regions so people instead port the script or retranslate the original. In that case you are still most likely grabbing something (we have seen languages hidden between regions but I have not seen any flavour of undub want something like this).