like damon stated, cancel would be the way to go but keep in mind that canceling a jump with normal cancel command will make you lose the jump´s movement/momentum, you will need to use jumpframe on the freespecial you are canceling in to in order to simulate that momentum, there are script ways to do a better work of this but i am not familiar with them, check this treads for further reference.
http://www.chronocrash.com/forum/index.php?topic=5838.msg77446#msg77446
http://www.chronocrash.com/forum/index.php?topic=50.msg2544#msg2544
http://www.chronocrash.com/forum/index.php?topic=2466.msg51865#msg51865
http://www.chronocrash.com/forum/index.php?topic=4676.msg64811#msg64811