Some of them. If you want to submit a game you've already made, make sure to read the rules of the jam first. A lot of them will say that the game must be made to fit the theme, but there are some, like the Decade Jam, where there is no theme and so long as it's been made in the past 5 years or so it's okay to submit. There are also some like HOW LONG CAN WE JAM where the jam ends in 1750 years and it doesn't even matter.