You are entering the world of hybrid setups.... basically your wtf meters will sky rocket from here on
You need an audio interface with great AD/DA converters with at least 8 inputs and outputs. I have a similar setup where I send my audio stems from logic to outboard eqs, comps and summing, then turn it around and send it back to logic.
For example let's say you want to mix your drums elements on your outboard mixer with an audio interface that has 8 in/outs (same concept applies if you have less than 8 ins/outs).
Output section
In logic's mixer select the Kick drum and assign it to output 1-2 instead of 'stereo out' (actually stereo out is output 1-2). If it's a mono track, just use output 1.
Move on to the snare and assign it to output 3-4
Hats to output 5-6
percussion output 7-8
Afterwards physically connect the outputs of the audio interface to your mixer inputs. By now you should be able to see levels on your board. If not, check your cabling.
Input section
After you have mixed all the drum stems in your mixer, physically connect the 8 outputs of your mixer to your audio interface inputs... in logic create 4 stereo tracks and name them KICKin, SNAREin, HATin, PERCin. Assign KICKin to Input 1-2, SNAREin to input 3-4, HATin Input 5-6 and so on...
make sure the software monitoring option is activated so you are able to listen and viola...
Hope this helps!