I am working on a game right now (a platformer type game) and I was wondering how to add multiple levels because it wouldn't really be a platformer with 1 level.

I believe platformers use the costume of a sprite as the ground/walls. You can also try to detect when the player reaches the end by using the x coordinates. When the sprite has moved to a certain point on the x coordinates (left to right), then you know that it's the end of the map. Then, you just change the costume to the new level and add the necessary UI, such as a “Level _” indicator.