You can go into the Modulation Matrix and then navigate to your assign 1-6 parameters. Select an open assign bank and then set the source to be FilterEnv, use the bank button to page over and set the destination 1 to be FltAttack, then bank over a couple pages and set destination 2 to be FltDecay. Now by adjusting the values for each of the destination you can increase the convexity or concavity of the attack envelope. Play around and if you are interested in a more descriptive definition of what is at play here, do a little research on the web on "recursive modulation"