The spirit of the jam is for people to make entirely new games. Making just a new level within an existing game isn't very compatible with that, nor is it very fair to the others who are making entirely new games.
Of course, people can submit whatever they want (we don't have control over that) and see what happens, but anything which tries to test the limits of contest rules should be fully disclosed in the description and, as you mention, may get disqualified.
If you want to participate in the jam, I'd encourage you to consider making something new for it. It might even be a nice break from what you've been working on!