It's a possibility, says creator Judd Apatow. This is what he posted on the official Freaks And Geeks message board:

If Freaks sells Undeclared will be released. It is difficult for the same reasons as Freaks, music clearances etc., but hopefully the success of the Freaks DVD will make people want other shows in the same genre. We have lots of funs deleted scenes for that show as well. It is not as emotional a show as Freaks, not as touching, but there were some comedic moments that I love, and we had some fun people on it like Amy Poehler, Will Ferrell, Adam Sandler, Ben Stiller, Kyle Gass. I think it will happen, maybe late this year or early next year. Cross your fingers.