how do you do it?? i.e., matching the color for the already colored fondant? or do you just mix in white gumpaste and kneed it really well? (can ya tell I haven't done this before?)
Why do you want to knead gum paste into Fondarific?
If your aim is to have the fondant dry hard or work like a 50/50 mix, you may be sorely disappointed. Fondarific/Duff's Fondant has a huge amount of fat in it--many times what's in regular fondant--and it doesn't respond much at all to the drying/hardening gums used in gum paste. It still remains pretty soft, apt to slump under it's own weight, and vulnerable to heat after adding tylose, gum tex, or cmc to it.
Ditto to BlakesCakes. I tried adding gum-tex and it made not one bit of difference. I swear I heard it laugh at me.