I bet there is a way to add it from GM you just gotta know who to ask (I don't). Everything in that truck is controlled by a computer including the idle, I mean the thing idles up in the morning here when it’s cold so why would you not be able to do it manually?
and if the GM fix is to pricy there are tons of rocks out in the desert that can be placed on the GO pedale. thats what we do with my buddys truck when we are welding on the trail! 
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(My Truck) Black 2008 Chevy Crew Cab Short Bed
4x4 Z71 LTZ LMM
Speedliner, WeatherTech FloorLiners, B&W Turnover Ball
(Work Truck/Tow Rig) Black 2005 Chevy Crew Cab Long Bed Dually
2WD LT LLY
Transfer Flow 100gal in bed tank, B&W Turnover Ball, Edge/Attitude Level 2 all the time, 4inch Turbo Back Exhaust with Cat Delete | 
Linear Mode
