Where is your mixing being done? If it's not being done in the radio, you can't reverse the servos in the radio. If the mixing is done in a receiver, 3-in-1, or the like, then the servo signals are not sent over the air, so there's no way the radio can reverse them.
You can reverse a servo electrically two ways:
1) You can buy an inline servo reverser.
2) You can electromechanically reverse almost all servos. This requires opening the servo, swapping the two internal motor leads, and swapping the two *edge* potentiometer leads. Do not touch the middle potentiometer lead.
If you're comfortable with working on small electronics and know how to solder with reasonable proficiency, reversing the servo is not too hard. Otherwise, shell out $10 for a servo reverser.
http://www.rcdude.com/servlet/the-19...everser/Detail