I have a Schertler Jam 400. I would like to be able to run more lines with it. Is it possible to use an 8 channel mixer? I would then plug the vocals and instruments into the mixer then plug the mixer into the Jam 400. Would I plug the "out R" and "out L" from the mixer into two separate channels on the Jam 400?
I don't see why this wouldn't work!
As long as you make sure that everything is in balance on the mixer so you won't risk overloading your amp, it's fine.
You can either plug a mono output in one channel, or as you say, plug the out R and out L in two separate channels, but then make sure that your L and R are in balance on your mixer and make sure they are balanced on your amp!
The 'hi Z' labeling on the jack inputs for channels 1-5 doesn't quite agree with the technical description in the online manual for your Jam 400. Try them with Line Out of an external mixer if you like, but start with the Gain control right down. It may be safer to use the two phono inputs on channel 6. Don't feel that they're somehow 'second class' for being phonos. They're electrically the same quality as the other inputs. Possibly even slightly better than the XLR mic inputs, as they can bypass a stage of amplification.