You don't really need a script for this - you just need to design the new maps so that they can be put into the project and used with the existing ones. How you get it to the player is your main concern (removing the map files prior to compressing, then adding them back later, means you can just get the players to add new map files to the same folders and they'll work). The bigger problem comes when you also want to add new stuff to the database, or more scripts.
Does that works on encoded database(rgss3a) too?
Some other games have mods that read files from folder as top priority before looking into the packed database.
So files in folder will replace the ones in rgss3a.
If that's the case, just ask the player to make a new folder named "Data" and place the extra map files in it should do the trick.
Should be able to replace certain contents this way too.
Database contents(actor/class/skill/item/etc) and scripts might corrupt existing save files so it's not recommended to alter those.