Does the MOXF8 have aftertouch?

No.

And, is there a way to set a curve for the expression pedal?

No, the Expression pedal is linear, 0-127, in nature.

What you can do is setup a maximum output level for the toe-down position of the FC7 pedal sending cc11 Expression... by setting the Part Volume controlled by cc7 to a specific setting, you are setting the maximum output level for cc11 in the toe-down position. For example, if in a mix you determine that maximum output level for the B3 Organ part is 99... meaning when the MIXING Part Volume setting is a 99 that is as loud as you ever need to get with the organ relative to the other instruments... setting the MOXF Voice Volume at 0 and the mixer volume for the organ at 99. This sets the maximum, toe-down output for your Expression pedal. By setting the Voice Volume at 0, the position of the pedal will determine the output level. The Part Volume determine exactly how loud that maximum is.

Conversely, you can setup a minimum output level for what happens at heel-down, it does not have to be zero. Here you use the Voice Volume setting to set the how loud the sound will be when heel-down is used, and your FC7 will increase the output from that level.

Using one or the other of these configurations allows you to set a comfortable range of volume change for your pedal. You can have one set up for standing another setup for sitting.

This is covered in detail in the following article from the LEARN area of this site:
MOXF Physical Controllers and Destination Parameters

Thanks. I didn't think it had aftertouch, but it's hard to find some things in the manual.

The expression pedal I'm using, not an FC7, has an adjustment to limit volume but it responds too quickly.

Thanks anyway,

Do yourself a favor, get the Yamaha FC7 Pedal — next time you want to reward yourself with a gift!