I understand your struggle, we have the same sound board with a hearback and had the same issue. The problem is your sound board does not have direct outs, and that is what the hearback really needs. I found 2 options to solve it:
As Cory said, there are insert to direct out converter cables available. You could get a custom snake made, buy them individually (about $6 per channel), or build them yourself. Just google insert direct out cable and you will get all sorts of info.
We actually ended up using a different solution because we had trouble only having 7 inputs available sometimes and we actually needed our inserts for some channels. We bought a shure auxpander, it gives us a matrix mixer with a direct out for each channel while still having a insert available for each channel. This allows us to group several signals into 1 hearback channel when we have a lot of instruments and still allows us to use channel effects like reverb or external EQ & most importantly compressors to handle the bass players that like to "slap" the strings sometimes
