Hopefully simple mod to Stereo Effect Switcher by Loop Master

[zakmetal]

Hey guys,
I am new to the forum and a very novice electo enthusiast. I am looking into buying a Stereo Effect Switcher by Loop-master.com. I want to add a "mix" knob to it, so I can mix the we/dry signal when the pedal/loop is on. This may be a relatively simply mod, or at least it seems like it. I my solder skills are decent, and I am comfortable performing the mod my self as long as I know what to solder/cut where.
Any help is appreciated.
Thanks,
-Zak-

[zakmetal]

Do I just need a simple pot added somewhere in the signal? 
Anyone ?