Importing from file / File reading

Hi, I want to allow players to import user-created custom models/textures to the game.
I want to create a “custom” folder inside my game package so the players can put their models/textures inside that folder and it will appear when they restart their game.
Basically, can I make my game read files and use that files in-game?

Note: The game is going to be single-player. I don’t need to worry about networking.