Hi Dien, there are many variants on reusing existing content. For example, there are people who take individual funny videos and stitch together a Funny videos compilation of their own. To me, that's a lot more work.

Anyways, if you're seriously thinking of re-posting songs after adding lyrics, be sure to check out YouTube's Music Policy page first. It will tell you which songs you can and cannot use: https://www.youtube.com/music_policies

Thanks
-Samir