DJ-Mixers often has a mic input. You can use that input as an extra channel, for a keyboard etc that has a line signal. But you'll need a line box that converts line signal level to a mic signal. If a keybard is connected to a mixers mic channel directly the sound will be distorted. So a line box or a di box converts the signal to the right level. The input accepts phono jacks and the output XLR. If your mixers dont have an XLR input, you can use a xlr to phono cable.
DI Boxes comes in many sizes and priceclasses.
The audio quality depends on what linebox you choose, The higher the pricetag the better the quality.
But a low cost Di box will work too. They go from 20 Euros and up. Just choose one that converts a phono line signal to a xlr mic signal. A passive line box will do.
So now that you have a keyboard/sampler/drummachine on the mic channel lets look on how you can overdub the existing mix with it.
If you for example use a sampler. The sampler will overdub the other channels regardless on the crossfader setting.
So for transitions, just play the samplers desired pad on the outro of the song you mix out from.
Then it plays back the sound you just let go of the song from the next channel.
If youre using a synth then the sounds will overdub the mix whenever you play the keyboard. This depends on the level of the mic channel however.
Using the mic channel for an extra 3rd turntable is not recommended. The mic channel is bypassed from the crossfader audiopath. The Eq section often lacks settings for the mid frequencies as well. So its harder to shape the quality of the sound. Also there are no upfaders on mic channels.
For adding extra elements ontop of your djmix, just hit the sampler on the new channel. Very helpful for transitions between different genres that has a varied tempo.