I currently have a large drum kit which means I use a lot of mics. If I had a sub mixer next to my drum kit in order to mix my drums without taking up most of the mic inputs of the main mixer, would I simply be able to connect the sub mixer to the main mixer by using the main outputs from the sub mixer into one of the inputs in the main mixer? These are both digital mixers by the way.
As long as the outputs aren't powered speaker outs, yes that's fine.
But if they are both digital mixers you may want to check if you have a digital out on the submixer and digital in on the other mixer to avoid the extra digital->analog->digital conversion.
Yes, a sub-mix for drums or keyboards is common practice. But do you have to close-mic EVERYTHING in the kit? Bass, snare and a couple of overheads might actually sound better.