It really depends:   some boards are expandable - new channels straight onto the busses - some not. The latter you are going to have to run the sub-board into either existing channels or onto an aux input. The former you will have two sets of EQ in series. The latter you will probably have to be careful about the gain since most auxes are 100 mv not +4 db. Either way you run two preamps (in effect) after each other and can expect to degrade the sound quality.

So if you do, use the sub-board for the channels you can afford to degrade - line outputs from guitars is a fave, others put all the drums (except kick and snare maybe) onto the sub-board. I would say NEVER put the vocals on the sub, you want them as clean as possible.

Preferred option - bigger board!