Well, I think I found a solution. It was there, but I'm a bit slow…
Go to the Advanced page, then choose the Control Map pane. There, you can find the Master Attack control. Assign it the Velocity source, and select the Invert checkbox. Edit the curve at will.
Now, playing softly makes the sound's attack play slower, playing harder makes it faster.
If you are controlling the Velocity XFade with a continuous controller (I use the ModWheel), and it is sending a high volume value, when you play soft the sound's volume takes longer to reach the maximum level.
However, it seems that assigning the attack control to a continuous controller is a better choice. Fine-tuning the attack can be done after recording, and a continuous controller is more precise than velocity.
Paolo