Hello everyone, complete newbie here! Sorry for stupid question, but are there any guides on modding data of pre-made characters? I want to resurrect several pre-made sims, add them to appropriate households and give them necessary relationships/jobs/starting skills.

Making default changes to a world's premade sims is generally a no. The worlds as they're setup to be on New Game are pretty airtight, it's for this reason that custom worlds are populated via save file.

But there's also no reason to do it that invasively- you can make all of those changes in-game using NRaas MasterController.
Then, if you wish, just keep a copy of that save before you begin playing if you want to reuse or share it as though it were a distinct version of a world.