Ok...about using another external pedal in the chain, you cannot assign the external pedal to one of the pedals on the gt5.
I use a Bad
horsie wah with my GT5, the way to do this is to use the loop function. You can set the GT5 to turn on or off the loop anytime you want, by going to "pedal assign" and setting the control pedal to the loop function.
As for the external analog pedal, set it to anything you want, but bearing in mind that it will affect the sound of you signal chain in the GT5, the level of the external pedal which is fed into the GT5 can be adjusted by accessing the loop function and adjusting the level of it.
Sounds a little confusing yeah?
If there's anything you need me to clear up, just let me know
Chris.