TB, as woodhog suggests, I've also added a factory spring to my stick radio to have a self-centering throttle. It's not to difficult. Internally and mechanically, the left and right sticks are identical, but instead of a spring, they have a simple rachet mechanism so the throttle stick holds it's last position. That's a typical airplane set-up. Get in touch with the manufacturer, ask them what parts you need to make your throttle stick self-centering, and I am sure they can ship you the parts.
This thread might help give you an idea of how easy the conversion is. Some companies prefer you send the radio in, and they will do the work for you, but it's really so simple, you can do it yourself in a few mins.
http://www.rctruckandconstruction.co...entering+stick