The biggest risk in adding another device in your signal chain is signal level clipping. I'll be brief and list the points to watch out for.
Do not run line-level signals into microphone-level inputs. A DJ mixer can and will clip a microphone input on a mixing board.
Do not use the secondary mixer to increase the signal gain going into your speakers. The maximum output of your DJ controller/mixer is enough to get the speakers to their maximum output capacity. If the system isn't loud enough with the DJ controller turned all the way up, then you need more system, not more signal gain.
Don't use your eyes to gauge the sound levels. Your DJ mixer's outputs are VERY unlikely to clip. It's much more likely to clip their inputs, and even more likely to clip the inputs of another mixer to which the DJ mixer is connected. In other words, turn up your DJ mixer until the speakers are as loud as you want them to be, regardless of the indicated level on the DJ mixer. Reducing the indicated level on the DJ mixer by using a secondary device to add more gain is simply providing yourself with a visual illusion. All professional-grade signal-level devices produce effectively the same maximum output level: Nominal +4 dBU, maximum +22 to +24dBU, equating to approximately 12VAC from the signal outputs. More than that would clip the inputs of any line-level device. (Considerably less than that is enough to clip a mic input.)
Amp-level clipping can be prevented in the internal processing, so is no longer a cause of driver failures. Signal-level clipping can't be prevented in the amplifier, and so the amplifier does what amplifiers do and it amplifies the clipped signal. This can do the same damage to drivers as amp-level clipping, but the limiters can't prevent it.
The best way to add mics to a DJ system is to mix them before going into the DJ mixer and add them to one of the inputs on the DJ board. Some mic mixers even have mic-level outputs, which can be connected to the DJ mixer's mic inputs.
In conclusion, if you choose to run an additional mixer between your DJ mixer and your speakers, take very good care not to clip the inputs to that mixer. Be aware that the speakers have a maximum output level that is unchangeable regardless of how much signal is sent to them. Further, keep in mind that the speakers have limiters and that the sound quality deteriorates when the limiters are active. It's best to have the indicators on your mixer tell you whether the system is close to maximum output rather than hiding that information by adding gain at another stage.