1- Introduction

For some 402-VLZ3 reviews, see my bookmarks.

Excellent: disassembling the mixer and inspecting the boards show easy access to channel 1 and ch.2 signal path (without cutting any PCB trace)  at the exact point where INSERTs are frequently found: after the low-cut filter and before EQ. 

Here's the logical schematic from the owner's manual:

This is a simple mod that should be accessible to every one and a good occasion to learn if you've never done any soldering before! You'll have to desolder a capacitor to make some room for the jacks, drill holes, solder new connections, etc...