Does it require to unren the game first before you can use your own scripts?
No, you can put it in the game folder even if everything else is just RPYC files, and even if they are stored in a RPA archive.
In fact, you can even overwrite a file that already exist into a RPA archive.
Lets say that there's a "characters.rpyc" file stored in the RPA archive, and you want to change it because it have things like
define mom = Character( "Annie" )
, that you want to replace by
define mom = Character( "My incestuous mother" )
. All you need is to create a file named "characters.rpy", fill it with a copy of the original with your changes, and put it in the "game" folder.
Depending of what you did, the first time you'll try to launch the game, Ren'py will perhaps complain about duplicated values/labels. But it's just due to the compilation of your new file. Launch the game a second time, and it will works fine this time.
Side note: Obviously, you'll still need unren the first time to know what's in the said script, since here you need to copy it integrally. But after this first time, it become useless.