That one will be tricky to do because during World Gen, the game checks whether tree generation can happen. There must be certain conditions that are true for this to happen. One of those being whether the block to spawn the tree on can see sky. If it can not see sky, then no tree will grow.
Unless you're willing to settle for running around in caves, lighting them up, placing dirt and planting trees you'd either have to find a mod that does that (or learn to mod yourself which probably is more advanced than what you were looking for) or use WorldEdit or something to copy ground above a forest, though that would only apply to the area you had edited, not the whole world.