You can not.
Have you considered using a 'custom keystroke' and 'custom format' scripts so you can dynamically change this property using the 'setAction' method of the field object?
I'm trying to convert a field from comb to not a comb inside a format event and vice versa. "getField('myField').comb = false; getField('myField').doNotScroll = false; getField('myField').charLimit = 0;" would be the logical way since getting charLimit when its unchecked is 0. The easiet thing to do is to set charLimit to 1000000000 but it seems the dirty way to do it, but it will work fine for my purpose.
I have no problem with a comb field created in Acrobat 8 Professional and opened in Foxit Reader 3.0.